No one is quite sure at what point Natasha and Clint got married. In fact, everyone is quite certain that the two haven't even realised it for themselves.
Tony is not the first to notice, but he is the first to comment about their relationship, if it can be called that, over dinner one night in Stark Tower. It's always been an unspoken rule that Natasha does not discuss three things: why she refuses to take any jobs in Moscow, what happened in the Red Room, and anything involving her partnership with Clint. Tony has already crossed the line by suggesting they take her to Moscow to see how she if she really can drink with the Russians, so when he asks her playfully why she's not wearing her wedding ring she chooses to launch a chopstick across the room instead of roll her eyes and insist she hasn't got one.
The chiopstick jabs him in the jugular so hard he's sure for a minute that he's dying. He doesn't ask again.
But he does keep noticing the strange way that they...well, compliment each other. Neither Natasha or Clint are anything close to a wholesome human being, neither of them will ever yearn for suburbia and they don't even know what a real childhood is let alone be able to provide one for any ninja children of their own. Yet somehow they work remarkably better out of the field than they do during a mission - perhaps the biggest shock of all
Straight off the bat, Tony notices in the aftermath of saving the world, they're drawn to each other without even trying. Either that or it's been so long that they don't need to try. He's not sure how they met, and no one will tell him, but he's starting to think it falls within the grounds of What Happens In Vegas Stays In Vegas. They pay attention to each other without even being consciously aware of it, as if the whole charade is just perfect timing and good luck, but they're better than that. It's not luck that Clint just happens to catch her favourite coffee mug when Bruce's eyes flash green one day and it startles her into dropping it, and it's not perfect timing that Natasha just happens to have the coffee ready for two people the moment Clint emerges from the armoury every morning.
There are no lies between them, either. Three days after he had officially met Clint Barton during the 'settling down' period after what they now referred to as the 'alien incident' they noticed that he seemed to be moving akwardly. Everyone asked him about it, but he said he was fine, and they heard countless injuries. Trapieze injury. Old football injury. Slept on the wrong side of a bed. There's nothing wrong with me. Too much crazy sex. They heard them all, and believed none, naturally. Natasha confronted Clint on the third day immediately after he was bragging about throwing his back out during a wild sexual encounter and asked him what was wrong. Without even giving Steve a second glance, Clint told Natasha about how he had landed on his arrows when he crashed through the window and that it's causing him back pain.
An hour later, while the five remaining Avengers sit down to show Steve the wonders of the Star Wars movies, no one is brave enough to comment on how Clint throws off his shirt and lays down on his front in the middle of the room, watching the television with his head propped on folded arms while Natasha sits on the back of his thighs and gives him a back massage. No one dares say a word, because while she's working wonders with her hands based on Clint's peaceful expression, she could turn and snap their necks in a second. Tony does wonder how long it took up to build the trust that Clint had for her, to let her so delicately have the power to twist his spine and kill him in less than a second and yet to so willingly allow her the opportunity. Tony knows that she wouldn't kill him without a good reason, now that he knows her a little better, but he doesn't think he could trust Natasha not to kill him in that situation. For him, the fear would still be there, but for Clint...well, Clint is asleep by the time they put Empire Strikes Back on, so Natasha crawls off his legs and lays out in a similar position beside him.
They did that a lot, too. Moved into each others space as if they had a permanent spot at each other's side with their name on. When they're on base at S.H.I.E.L.D waiting to shower after a training session, Steve has witnessed first hand how rather than waiting for an empty cubicle, Natasha will just whip off her clothing and waltz into the one Clint is using, and he's heard the playful squabble that follows while they fight for the hot water. Clint allows Natasha to stand right behind him, almost teasing him while he stands in the armoury and lines up his bow for another impecible shot. Natasha allows Clint to lean halfway into her chair when she reads the morning papers over breakfast. Clint allows Natasha to creep into his room in the dead of night when she's kept awake by thoughts of...well, no one knows that part, but they've all heard Natasha shuffling through the hall, Clint's door opening, and the archers voice simply saying 'come on, Nat' in a soft invitation before the door is closed. Natasha allows herself to do that.
Sometimes they went days without sharing space, though, usually when one of them was irritable. Clint was told that he wasn't cleared for any further S.H.I.E.L.D operations until he had completed a mandatory psychiatric evaluation after being compromised. He had to requalify for his Agent status in full and spent a full week slamming doors and glaring at anyone who went near him until the following Thursday, when he slammed a coffee mug down, breaking it at the handle and stormed from the room just because Fury was calling his cell phone. Natasha had calmly put her own coffee mug down, folded the paper and followed him down the hall. The sounds and the argument that followed told them all that Natasha had overpowered him, and they peeked around the corner a few minutes later to see that she was calmly holding Clint in a thigh hold while he promised her that he would calm down and stop slamming doors. She kept him there for a full hour, never relinquishing her borderline-dangerous grip until he also agreed to stop being pissy about work, to actually clean up after some of the meals he made, and to bring her coffee every morning for a month.
They have a silent appreciation for each other, too. Natasha's certainly not a Rachel Ray in the kitchen, but she makes sure that she knows how to cook sweet and sour chicken, just because it's Clint's favourite (Tony knows that's the reason because she only ever 'coincidently' makes enough for two) and on these nights while they're watching a movie he'll pull her feet into his lap and rub her feet, even though his hands are far from perfect when it comes to a massage. Too rough. They all get the feeling that she prefers it that way, though.
They communicate perfectly, not always verbal or even physical. Just a look, a tone of voice, and they will have made a joint decision before anyone else in the room realised that there was a need for one. They speak far more languages than any other agents, except perhaps from Fury, who confiscates the childish notes they pass each other during debriefing and scowls at the words. Tony starts trying to find out what they've written, but is never fast enough to get a look before Fury gets there first or Natasha tries to stab him with a pen. He decides that she's writing filthy things to him in Russian and from the look of Clint's handwriting he's responding in some kind of heiroglyphic system.
When they argue, its a warzone. Steve goes to the gym and only returns to eat in his room, Tony spends days in his lab and convinces Bruce that it's a good idea. Bruce believes him, because he only stays to witness the first day of their argument then retreats to Tony's lab to hide away. Even Fury doesn't set them straight, but admits to Steve one day that they argue more without Coulson to talk sense to them. Apparently his relationship as handler to the two agents was every part of the nanny nickname that he was given. Natasha is silent and deadly, all glares that could cut steel and an aura of hurt and anger that could make your blood run cold. Clint is the opposite, he's loud, and he's the first of the two to shout when they start disagreeing.
Their tempers don't compliment each other, but they are pretty good at the apologies once they've calmed down. Once, after a particularly bad argument, Natasha coaxed a squirrel onto the terrace to entertain Clint and the two of them spent hours on the roof baiting it with varies tidbits of food from Tony's kitchen like two children with an injured bird. They still see the squirrel sometimes, and Tony is a little surprised when Clint sees it at the window one day, nudges Natasha and announces that their baby is back.
When Tony comments on their, what he has has decided to be, marriage, he's not surprised that the others have noticed the same, but he doesn't quite see what a true relationship the two have until Steve and Bruce point out what they've observed as well.
Steve has noticed the Natasha has stopped drinking coffee that the three make her, only accepting coffee from Clint. She forces Clint to celebrate his birthday, to remember his mother's death and visit her gravesite (never his fathers or brothers, she knows there is bad blood there). She knows exactly how much alcohol he can handle on a bad day and when to cut him off before he starts reliving his past. She knows the signs of him being too cold on a rooftop during a stakeout long before he'll even admit he needs a refill on water.
Bruce notices that Clint will never Natasha win when they're sparring, but that he will never strike her in any way that one might relate to domestic violence, that Clint will always hover around the infirmary for 'unknown' reasons when Natasha is there, that he will always spend the night in her room afer an infirmary trip, from the time she popped her shoulder out of socket to the time she got a just-slightly-too-deep graze on her temple during a mission. Clint makes her breakfast every Sunday morning, and the one Sunday since they began living together than they've been apart Clint made himself extra breakfast, two coffees, and glared at anyone who attempted to take the extras. He had the whole meal himself, drank one coffee immediately, and held the other one for the next half an hour, brooding and miserable as he sipped at it the same way Natasha usually did.
Gradually, there are more changes. During a part of a remodel of the building Natasha started sleeping in Clint's room and never went back to doing so. When Tony arranged for her things to be moved into the new wing just for her, giving them all their own space on the above floors, she bruised his sternum and transferred her few belongings, enough to fill her arms, into Clint's area. Clint never complained, just made a few adjustments to his bathroom shelves to store their different shampoos.
And then one night, over pizza, Clint sets it in stone.
"What are we doing for our anniversary this year?" he asks her casually.
Bruce and Steve look up, exchange glances and stare very hard at their own meals, but Tony is grinning with half a slice of pizza hanging from his mouth.
"I knew it!" he said, with a mouthful of melted cheese. "You're married!"
Natasha glares at Clint. "Now look what you've done."
Clint shrugs. "We always celebrate it."
Tony finishes the pizza, which is a struggle because he's clearly too excited to chew his food properly. "I knew it! You're completely married! Tell me, was it a June wedding? Or was it a proper trashy Vegas?"
Natasha turns her glare to Tony, then drops it and resumes eating. "We're not married," she tells him simply.
"Yet your anniversary is coming up," Tony says, noticing the quiet look that is passed between them.
Clint is silent, looking at Natasha and they're having another one of those silent conversations that annoys Tony oh-so-much until Natasha resumes glaring at her food and Clint continues. "We're technically married, but at the same time, we're not."
That's not the answer he expected. "How can you be married, but not married?"
"Paperwork error," Natasha tells him simply.
Clint shrugs. "We went undercover, had new identites arranged and were supposed to be a married couple. In depth cover. We finished the job, came home, and found out that one of the newest recruits who had been arranging the cover actually applied the 'married' part of it to our S.H.I.E.L.D records."
Steve looks between the two of them, and Bruce nods slowly. "Oh, I see. Because of how deep the cover was it was legally binding."
Clint nods. "Tasha nearly killed the guy."
One look at Natasha's face right now, and it's not so hard to believe.
"Clint decided it would be a good idea to leave it in place, since we'd actually have to go through a divorce process to change it." Natasha spits out, glaring at Clint, but he's grinning like he's seen this reaction a thousand times.
"So technically, we're married," Clint said. "We celebrate anniversaries and we sometimes have sex...that's about it. No rings, no flowers, no I-love-yous," Clint looks at her sharply for a moment. "Right?"
"Right," Natasha insisted, making a face as if someone had offered her a dirty rat to eat with her meal. "Love is for children."
"Exactly," Clint agrees, turning back to Tony. "So...that's it."
And that was that. Legally, they were technically married. In practicality, they were incredibly married, but from their own perspectives, they were just normal people who occassionally had sex. There are no more questions, only because Natasha's glare continues to grow so much that they're all in a hurry to leave the pair to their fake anniversary plans.
Later that night, Bruce is the one who is on his way to the kitchen to make himself his usual evening tea, and he is the one who sees them standing side by side, Clint washing, Natasha drying, putting the dishes away afterwards. They're talking in hushed tones, arms gently brushing as they go about the domesticity and he stands for a moment to observe the two assassins in a simpler habitat than they'd have normally allowed him to see. He can't hear the words, but every few moments he hears a smile in Clint's mumble or a smirk in Natasha's whisper, and once they both laugh softly together at something he can't even imagine. He remains in the shadows as they finish the task, Clint dries his hands with a cloth that she hands him then places both his hands on her cheeks, placing the most innocent of kisses on her forehead, lingering for too long after for them to agree with the act they put on. He does hear the conversation after this.
"Love you, Nat," he mumbles against her forehead.
"Love you, Clint," she whispers back.