The first time they have sex, it's literally an accident.

They'd been working side by side for weeks, compiling a body of work and research that would have taken other teams years or decades. Bruce's training and experience combined with Tony's skills and resources have proven a fine partnership.

With their shared considerable intelligence or propensity to genius as Tony might say factored in, they'd made more headway on Bruce's big problem and some of Tony's smaller ones than either had been able to achieve on their own.

And they'd both liked doing science together and gotten on well and gotten to be even better friends, and the whole thing was going rather splendidly and ordinarily (for them) when they accidentally have sex.

Maybe they had gotten too cocky really, not wearing the safety equipment that would have been required for grad students that day in the lab. The last few days they'd made headway in several experiments involving tranquilizing and emotion-altering serums, and the first round of tests on the rats had been promising.

Anything that might slow down the Hulk once he was turned was welcome to Bruce, and it had been Tony's idea that the monster's mood could also be altered with the right chemical equation.

"Imagine, if you will, happy Hulk," he'd said, spinning a glass beaker counterclockwise on the tabletop. "Or super-focused Hulk, ADD-cured Hulk. Cuddly, humanity-loving, rescuing-kittens-from-trees Hulk." And Bruce had snorted but let Tony order up a bevy of fancy and exotic elements to experiment with.

"We'll cook the big guy an antidepressant that'll make Big Pharma and Hollywood weep simultaneously," Tony promised.

And they'd been cocky about the early signs of success, so that Bruce is being an idiot and isn't wearing anything more than safety goggles bent over the centrifuges in the lab with Tony across the room bent over the rats.

That's when one of the construction teams in the basement accidentally hits the mainline, and Stark Tower, for all its safeties, flickers out and dark faster than it takes to blink. In the next blink the lights came back on, and the power surges, and the tube Bruce has been halfway inserting is snapped up by motion and smashed.

Liquid and vapor are spun at him, flung at him, and then he is stumbling back, tearing at his coat, heart beating much too hard. Never a good sign. Knows the way this kind of thing usually goes.

Tony's hands on his shoulders, at the back of his neck. Tony's voice, steady, saying, "It's okay. It's okay, Bruce. Tell me. Which tube? Which one?"

There are many options, many emotional states they've diluted down to their essence, and some are better to have been sprayed with, and some, for Bruce and Tony and the Hulk and Stark Tower, are far, far worse.

"The - the -" Everything in Bruce is shifting, altering, his blood rushing, his instincts no longer his own. "Oh God, I -"

"Are you changing?" Tony's strong hands help rip away his coat, his compromised shirt. His fingertips skim Bruce's collarbone as though trying to detect a sudden surge in green underneath the flesh. Bruce groans. Tony's voice is not frantic, never frantic, Bruce appreciates that about him. Tony's voice is crisply cool. "Banner. Are. You. Changing?"

When Bruce's response is to gasp and flutter his eyelids and start to shake as the first crest of it rides him he hears Tony tell the air, "JARVIS, I want my suit brought up to Dr. Banner's floor and four teams positioned both above and below the main elevator. Stand by. "

"Yes, sir. Do you require-"

"No," Bruce is able to rasp out then. "No, Tony. I'm not - I won't - I -"

But he's still tearing at his clothes so he's probably not imparting the highest level of confidence. His hands are on his buckle and nearly part the zipper from his fly and then he's shoving all of it away, all the clothes are blessedly gone.

He's standing bare-assed naked in the lab, and maybe now Tony will understand what no words could quite convey. Bruce is so turned on he could nail nails with his dick, which had swelled to its full impressive size immediately after contact with the solution.

He's bigger than usual, even, totally engorged with rushing blood and all-encompassing need. He snaps his mouth shut and lets his trembling hand attempt to offer some relief. Has several milliseconds ago decisively stopped caring about the presence of Tony Stark and his ethereal robot butler.

Tony is watching, head tilted, scientist's curiosity combining with his best in rakish grins. He no longer looks uncertain, just observes. "Ah-ha," he says. "So it was the pheromones."

Bruce bites out a groan. Since the fucked-up cat's out of the fucking bag - "Feels like I'm gonna die if I don't come," he manages, somehow meeting Tony's gaze. He knows that most of him has flushed crimson. Better than green at least.

He's so hard in his own hand it's alien: it's been longer than Bruce can remember since he was so aroused. Isn't sure if he's ever been. Get a facefull of human hormones diluted to their essence, though, and -

It doesn't hurt matters to have Tony Stark staring at you like that, like you're fascinating, and dangerous, and delicious. Unexpected, and not. More unexpected is the means and methods to this, not the end result.

Because Tony says, "Christ, and for a minute I thought we had a real problem on our hands. Good show." He drops to his knees, flicks his eyes up at Bruce just the once, then takes the entire length of him in a go. One swallow. It's show-offy, and impressive, like everything about Tony, and perfectly explosive, like one of Tony's weapons.

Bruce grips the edges of the table so that he doesn't lose his equilibrium entirely, and Tony swallows his cock like he's done it a thousand times which maybe he has but this is their first time. Bruce is on fire, his heart pounding at double speed but in a different fashion entirely than triggered the Other Guy.

His blood is pooled in his groin, all of his taut body thrumming with sex sex sex sex which at least was far better than hate and Tony is hot wet heat around him. Never let it be said that Tony Stark is not amongst the world's finest cocksuckers, Bruce thinks through haze.

Tony kneeling with his mouth suctioned on Bruce's cock is a beautiful thing, Bruce thinks when he tries to think clearer. His full lips are stretched and for once his clever tongue is too preoccupied to talk. Bruce likes watching the way the length of him slides in and out, in and deep. Tony's head bobbing, half in-charge of the operation and half supplicant, on his knees.

After a time Bruce's hands push off from the table and he steadies and Tony's bright eyes are watching him. Bruce's fingers thread through Tony's hair and curl firmly in his short-hairs, and with a thrust of his hips Bruce changes their motion.

Has Tony framed and gripped, starts to slow-fuck, then fast-fuck his mouth, tries to see how much he can take because. Tony's eyes on him are wider but the edges of his lips curl up, and Tony can take a lot.

It's too good and too intense and Bruce can't last much longer but he tries, finds himself trying, balling up his fingers and toes like an attempt to keep off the Hulk.

Easier to resist the Hulk than orgasm with Tony Stark giving him the blowjob of a century, though, and so the roll of Bruce's hips speeds and he comes harder than he has in years, in so fucking long, comes with his cock halfway down Tony's throat and Tony everywhere around him.

Tony lets him slip free when he's clean, can't resist adding a visceral smack of a sound, like a gourmet meal has been had. Bruce's knees are weak, but he finds he can return Tony's smirk. Slides bonelessly to the ground to sit beside him on the floor of the lab.

"Whoa," says Bruce, finally, deciding on a noise-word for it.

"That," Tony agrees.

He's spectacularly aroused through the thin fine linen of his pants, and Bruce finds it an easy thing to reach over, palming the fabric. After an extended period of righteous face-fucking a little feeling-up isn't much to blush over. Still, Tony arcs up against his touch, makes a satisfying sound, like Bruce has surprised him. Doesn't do anything more than watch him with sharp eyes after that.

Bruce says, "Can't I return the favor?"

Tries to say it lightly, light as his touch on Tony, tells himself the reply shouldn't hurt if it does. All of this an accident and yet somehow expected. Impossible. Improbable. Then again, so were they.

Tony says, "Is fucking you, like, an option here? Is that on the table?"

"Prefer a bed to tables," Bruce says, as though they've already bypassed a good bit of answer, which they have. "I bet you have awesome sheets."

It's true. Tony does. He is not modest about it. He shows Bruce. They show each other a wide array of things that are awesome.

The second time the have sex, and the third, and the fourth, are not accidental.

The eighth time, it is Bruce who fucks Tony, pinning him down onto the leather of a couch in Tony's elaborate study. Bruce is heavier than he looks, he finds. Can hold Tony down. Bigger on the inside.

Bruce forgets what exactly sets it off, whether it's something Tony says or a cant of his jaw or a particular smile or something in the way the muscles move under his thin t-shirt, but Bruce climbs decisively over him and stays there.

Kisses and touches and sucks and rubs and pulls and scratches and bites and licks at Tony underneath him, until Tony puts his head back and edges his thighs apart and opens up for Bruce's fingers.

His teeth are too clenched, though, and he's tight enough that Bruce knows it's been a while, a very long while, since Tony had let himself be taken like this. Bruce kisses him some more after that and is even more inquisitive, more careful, treating Tony's wondrously bared form like a controlled experiment. Everything is tested and retested, then every part of Tony is tasted and retasted.

When Bruce has him panting and squirming on the leather and as close to begging as begging could be had from the man who had everything, Bruce gives him what they both want. The one thing Tony can't have on demand. Bruce pushes into him all slicked-up and gentle, fits them together long and slow with their foreheads pressed firm.

He's pointed and angled perfectly and hard as fuck as he fucks Tony, as he kisses Tony's mouth while he does it. It's intimate and strange for them, this much kissing at least, and some of the noises that Tony makes are new.

His arms banded with muscle finally slide up and over Bruce's neck, and his legs wrap around to pull Bruce deeper, and that's the signal that Bruce can give in and screw Tony the other way they both want: with total abandon, tapping into the dangerous vein of adrenaline running through Bruce. Close but never too close.

Just enough to nail Tony hard enough into leather with his cock to make the couch's joinings groan louder than they groan. Hard enough to make them ache and plead and grip at one another and sometimes hard enough to block out all the other things that they are, which is always a blessing.

The eleventh time, Tony has Bruce up on one of the lab's many tables, cool steel against his skin where Tony has torn at his shirt. Papers are getting crinkled beneath them and they're perilously close to upsetting a set of full graduated cylinders.

"We can't keep doing this," Bruce says, and at Tony's eyebrow hastily amends, "I mean we need to set some limits. Daytime limits. We've lost at least 2.5 research hours to fucking this week already."

"Hmm," says Tony, his hand expertly fisting Bruce's cock while his other hand works his pants down. "No. I mean, yes, I see your point, Dr. Banner, but I dislike rules. Think of it like lunchbreak if that makes you feel better. You can clock me in," Tony says before he has Bruce's ankles hooked over his shoulders and the conversation turns to better topics.

The sixteenth time is different and starts out poorly. Bruce slips into Tony's office, using overwrite codes he'd decoded on the lock-out mechanisms. Stark security was impressive but not impenetrable when one knew what to look for.

Intent on surprising him with an energetic blowjob to celebrate Tony's return from a two-week tour of the Middle East, Bruce is surprised to find himself beaten to it. Well, the blowjob part at least.

Tony is in one of his big high-backed chairs, pants shoved down around his ankles, the golden head of Steve Rogers buried between his thighs. Steve, balanced on one knee, hands slack, is mouthing inexpertly at Tony's cock. It isn't exactly energetic, but it's happening. It's still happening when Bruce knuckles hard at his eyes.

Tony has his hands twisted in that minted-gold hair, is murmuring low encouragement.

Bruce is too close to leave undetected by two superheroes so he clears his throat, tries to make it a soft sound. "My, uh," he says. "My business can wait." Tony's eyes are on him immediately, trying to say hey wait just a sec you in a series of elaborate morse-code blinks.

But Bruce is backing up, and Steve does not look up, only freezes mid-lick and reddens. Bruce keeps backing up until he hits the wall and leaves, breathing hard through his nose so that he doesn't slip and destroy half the building.

An hour later Tony's at his door, fresh-shaved, fresh-showered, soft in silky pajamas. He has cookies from the airport in Dubai tucked under his arm.

"You have got to be kidding me," says Bruce, surprised into talking, which he'd not intended to do. It wasn't like they were - wasn't like they were exclusive, or anything, just that lately they'd been spending more days and nights together as not. It wasn't like they had a lock on each other, only that they hadn't had secrets and locked doors before.

Life wasn't exactly traditional at the Tony Tower as-is, which was the way he liked it and they all liked it, but Bruce thought they'd developed a sort of etiquette about it at least. Like, if he'd decided to casually fuck one of their teammates Tony didn't already know about, would that be okay? How would he feel walking in on Bruce taking it sideways from Thor? Thor. He should see how Thor was doing...

Pacing's bad for his Hulk-heart so it's almost better to face Tony in silken night clothes.

"You're painfully adorable when you're jealous, you know," Tony says, whisking past, before Bruce can seize and crush and kill him. "It's a good look for you. Christ, I wanna fuck you so bad. It's all I could think about crossing the Pacific ocean."

Bruce tries to remain unmoved by this. "And along the way you thought you'd show your appreciation for our armed forces," he says.

Tony shrugs it off. "What can I say, I'm a patriot. I love my country." He breaks out the cookies and busies himself mixing up too-strong cocktails at Bruce's well-stocked sidebar. Tony had been the one to insist on it.

He passes the drink into Bruce's hand, clinks their glasses together and takes a big first sip with raised eyebrows. "Banner, the kid came to me all fucked up about shit and I'm not good at the psychology mumbo-jumbo. Somewhere in his rant I got that he wanted me, or wanted to try me out at least, so I let him have a go. He's all trapped in pre-Kinsey psychosexual melodrama." Tony cants his head, makes his dark eyes round in an innocent-brilliant fashion that had wooed many a woman and man and dictator. "You forgive me, right? Wouldn't you have done the same?"

"Would've done it with more finesse," Bruce says. Despite his mood mere minutes before, he nearly smiles at the thought, and when Tony sees the flicker of that he knows he's won. He pads over to press a cookie on Bruce and then a proper greeting kiss to his mouth.

"Haveta admit," Tony says, when Bruce starts to divest him of the recently put-on pajamas, "When you first saw me with Captain Stevie, I thought for half a second I'd be seeing the Other Guy."

"For half a second," Bruce admits, hands reclaiming Tony's skin, "So did I."

"Huh," says Tony.

Other people would run away screaming exactly at this point. Many other people. Most other people. Normal people. "You really are adorable puffed up like that, all green-eyed only with jealousy. You get this little crinkle in your brow, just so," and Tony goes on talking until he talks them backward across the room and back into bed where they belong.

The next night Bruce phones Steve. He's the only one who answers his landline in the Tower and prefers the old-fashioned communique. Answers before the end of the first ring.

He tells them both to show at a precise military hour so they arrive on time. Steve is red again under his golden locks. Tony is defined by the towering, questioning heights of his eyebrows.

They eat a sumptuous feast together punctuated by a lot of liquor and when they're all ready for it Bruce and Tony show Steve the best night of his theoretically youthful life to date. They're so good together at this point. Makes this kind of thing easier.

It's easy to extend that generosity and excitement to Steve, and with the three of them it's thrilling and everything gets to be exhilaratingly new instead of weird. Steve, once relaxed, proves pliant and then not inconsiderably able, as he is in most undertakings.

Takes Bruce's cock deep into his mouth while Tony rides him from behind or varied combinations of the two that blur into threes and then just sweat and rhythm and intertwined limbs and all the things friction and bodies pushed together could do.

Everything is perfectly beautiful and everything gloriously hurts, from all of the rough sex.

Eventually Steve lies shiny between them, nestled comfortably. "Whoa," he says.

Bruce and Tony share a proud, meaningful look over his fucked-out form.

"That," they agree.

The twenty-first time, Thor finds them in a broom closet.

Why the demi-god needed a broom Bruce will never know, but he swings open the door, one of the few in Stark Tower that still had hinges, to uncover them in a considerable state of undress and disarray.

Thor blinks. Bruce blinks. Tony whines, "I hate entryways without mechanical overrides."

Thor peers closer. "Is this a thing that is secret? Why was I not told of it?" This could be construed as a contradiction in terms, but he narrows ice-blue eyes and crosses forearms big as mountains and waits for an answer.

"Not - not a secret," Bruce says, finding himself speaking for them, what they'd agreed on. "Just something we weren't shouting. Makes for targets, you know, big guy?" Likes to be able to call Thor that because Thor is one of the only amongst them who can face the Hulk without fear or special protection, and Bruce has always respected and feared that about him.

Thor considers them, and after a long moment nods sagely. "You have made the correct decision. It is better not to speak aloud of such personal entanglements, as we maintain amongst the Asgard," and he bows a little bit and closes the door of the broom closet.

Tony blows out a long breath. He still has hold of Bruce's cock, and Bruce is laughing and can't stop. "We're fucked," Tony says.

"Totally fucked," Bruce concurs, pushing him back amongst the dust-bins and bristles, then proving it.

By the next day everybody knows.

Natasha and Clint stop by together, and none of them go outside for two days. After that they visit sometimes, on a special occasion, together or alone.

They take Pepper on a weekend jaunt to Milan and raid half of Italy's fashion houses. They eat chocolate whilst skiing at Klosters in Switzerland and relax in the hotsprings of Budapest and return all of them the very best of friends.

After the thirty-seventh time, Bruce passes the entire night asleep in Tony's arms without waking once.

The forty-first time, they are exhausted and in post-battle agony. They don't want to think very much about the battle.

Their immobilizing serum had worked too well: Bruce had come back to himself mid-fight, found himself weak and useless in torn rags. Tony, and not Tony, a man in a gleaming iron suit, had plucked him from the chaos.

Stood him up in the rubble with the world exploding around them and said through a speaker, "You have to get angry again, Banner, it's the only way," and when Bruce shook his head the iron man shook him too hard.

"No?" Tony's voice was almost unrecognizable through metal. "Gonna hide out from the tough stuff with your balls between your legs, little man?"

The goading didn't work, the serum was too strong, they'd worked on it together and it was working. Bruce didn't have to turn into the monster. He was away at the moment. Bruce was safe, sound, whole, if dizzy, Tony's grip burning into the fabric of his shirt.

Then those glowing eyes had looked straight into Bruce's, and something like Tony's voice said, "You disgust me, you know. A monster and a coward. I can never decide which is worse."

Bruce flinched, but the gaze was electric, unyielding, the tone metallic. "You think I'm going to want to touch you now that I've seen what you are? You think I'll ever let you touch me again?"

And something terrible tore in Bruce and tore through him with a self-splitting roar and then the next thing he knows he is waking to being borne on a robot stretcher through the glittering halls of Stark Tower. When he opens his eyes he recognizes the type of environmentally efficient light overhead.

The stretcher is headed toward Tony's room, and Bruce moves, finds nothing groan-inducing in his musculature, looks around him. Tony is keeping pace nearby, with only a small limp and not too much blood on him. Enough of it, though.

"How'd we do?" Bruce coughs.

"We won," Tony says. "You - he - he was magnificent. Turned the tide. Saved the day."

"I'm glad," says Bruce, quiet. There's no need for noise anymore. His ears are still ringing.

"There'll be a parade," says Tony. "Get excited."

They reach the entranceway and Tony's hands are closing over his forearm to help him up. The robotic paramedics whiz away. Tony doesn't let go, just guides him inside, his long fingers encircling Bruce's wrist like a bracelet, or a handcuff.

When they're far enough in Tony noses cautiously at Bruce's neck. "You know I didn't mean it," he murmurs. His eyes don't try for puppy-dog roundness, stay serious.

Bruce doesn't move to object, so Tony starts to strip him free of the clothes they'd put him in. Bruce is unscathed in the low light, but when he reaches to peel away Tony's t-shirt there are many bruises underneath, mottling pink-blue-purple.

"I didn't know how else to make you do it, and we needed you. Him. I'm sorry."

"I know," says Bruce, to all of it.

"I thought about the worst thing in the world, and said it," Tony says. "I figured it might piss you off, too."

Bruce takes a breath. Lifts an eyebrow, crosses his arms over his non-super-heroically-built but perfectly nice chest. "The worst thing in the world was not touching me again. And you knew that I - and the Hulk - would agree."

"Yup," says Tony, and pulls him in for a lip-crushing, teeth-smashing, tongue-sliding, air-depriving firework of a kiss. The forty-first time, they've never been so battered and near broken, nor ever quite so desperate to put their pieces back together.

The forty-third time is the next morning. They have a meeting with the rest of the team in less than an hour, but they do it anyway, taking their time.

They decide not to shower, sharing a grin about the affect the heady scent of sex and their tousled heads will have on the debriefing.

Bruce is about to head to his own level for fresh laundry but Tony's hand on his stops him before he can slip from the bed.

"Stay," Tony says.

"Shirt," says Bruce. "Pants that aren't torn."

"I, uh," Tony begins, then says the rest in a rush. "I took the liberty of ordering you a wardrobe along with Pepper's when we were in Italy. It's here now. In the suite, I mean. My suite. You have a walk-in closet. That's yours. If you want. So you could just stay here. When you wanted. If you did."

Bruce looks at him, and they look at each other. He eases back into bed soundlessly.

The forty-fourth time is pretty loud, however, and makes them late to the assembled Avengers and their collective raised eyebrows.

Bruce is in a tailored suit of Italian linen, his hair staticky with his and Tony's friction, and their side-by-side entrance is fantastic. He doesn't remember a word of it, but thinks the meeting is the best they've ever held, and later that evening, they all turn the conversation to better topics.

After that, Bruce stops counting. He's always been better at doing science than math.