Written with love for doctorjamwatson, who said there was no way she was ever going to ship Stark Spangled Banner, to which I replied "challenge accepted."
Chapter One: In Which Phil Coulson Has Had Enough Of Tony Stark's Bullshit
Stark is the one who starts it, obviously.
After the way he flirted shamelessly with both of them, everyone on S.H.I.E.L.D. staff expected a love triangle, expected conflict, division, and eventually heartbreak when Tony was finally forced to choose, when it was made clear to him that despite what his dick wanted he could not, in fact, have both.
Nobody expected Steve to start hanging out in the lab in his spare time, asking questions and laughing when Tony goaded Bruce into joining him in teasing Steve lovingly (but relentlessly) for finally coming out of his technophobe shell.
Nobody expected Bruce to start loitering around the gym, munching on blueberries or granola and watching Steve and Tony spar with a lazy, content smile that brightened when Tony hung over the ropes with a flirtatious grin and wink to ask if he wasn't SURE he wanted to go a round or two, while Steve leaned in the corner with a look that would have been called a smirk on a less noble face.
S.H.I.E.L.D. was, naturally, divided on the subject. Those who supported Bruce/Tony insisted that Steve's sudden fascination with the lab work was just him trying to be a good team leader, making an effort to show interest in the important work his team members were doing. Those who supported Steve/Tony maintained that Bruce's appearances in the gym were part of his efforts to keep the Other Guy under control by exposing himself to a stressful, pseudo-violent environment.
But, since nobody knew for certain, speculation ran wild as the entire organization of Highly Professional Adults waited, holding their breath, to see which way the ball would roll. Natasha and Clint, who remained publicly neutral, were at the center of an intense, hotly brutal, and very profitable betting ring. Director Fury very deliberately turned a blind eye to the whole mess (although, after what happened to Stark when he made one too many eyepatch jokes last month and Fury finally snapped, nobody used that particular idiom anywhere there was the slightest chance he might hear).
The whole ridiculous mess finally came to a head when Coulson—the only one in the whole place who had managed to keep a clear head—got fed up with people being so distracted by gossip they got behind on their paperwork, and pulled the two superspies-slash-ringleaders aside for a scolding.
"Hawkeye. Black Widow. A word."
The small throng of eager betters vaporised in the face of Coulson's disturbing calm. Clint and Natasha traded nervous glances as they followed the deceptively placid Agent into an empty conference room.
A long pause followed, in which two ruthless trained killers were devolved into anxious teens by folded arms, raised eyebrows, and a patient, "I'm-not-angry-I'm-just-disappointed" stare.
Clint broke first, trying to dispel the tension with a transparently halfhearted cheeky grin.
"Hey, Phil, what's on your mind? Listen, if this is about that thing with the jello in the coffee machine, we had nothing to do with that, I saw Thor in there earlier and you know he's got a temper but—" His babbling was cut off by Natasha driving a sharp elbow into his ribs.
Coulson pinched the bridge of his nose as he replied in Controlling My Temper Tone #17.
"No, Barton, this is not about the coffee machine. I could not care less about the coffee machine. In fact, I could not care less about ANY of the nonsense you two get up to in your down time, although maybe if Fury would actually LISTEN to me for once instead of just shouting at me to get you under control, and remember that people with your skill sets and backgrounds need work and stimulation and that maybe it would help you act like adults if you weren't forced into this sedentary lifestyle with one mission every few months…I mean honestly what does he expect, you're going to keep your abilities honed one way or another, chewing me out because you're getting restless and taking it out on the coffee machines isn't going to change the fact that he won't give you any real work to do…"
Coulson trailed off in the middle of his rant, visibly composing himself as he heaved a deep, exasperated sigh. Both assassins shifted uncomfortably, shamefaced and avoiding each others' gaze.
"Phil, we're really sorry," Natasha began. "We were just messing around because we were bored, we didn't realize it was getting you in hot water with the director. The prank war is over, no question, and we'll even stop tailing Agent Hill for blackmail material—"
"And we'll stop stealing Fury's pens," Clint cut in. "And sneaking sex toys into Thor and Jane's rooms. And trying to break into Stark's private labs. And reprogramming the speaker system to blast dubstep in the middle of briefings. And—"
"Enough, enough," Coulson stopped him with raised hands and a tired smile. "I told you, I don't care about that stuff, and I can handle Fury. But this thing with Stark…" His face grew serious. Natasha and Clint traded looks. "It has to stop. You've already taken a small fortune in bets, and I've had it up to here with the gossip and distraction. I know you've been fueling the fire, and I have officially run out of patience. End it. Now. Today. I don't care how, as long as I don't hear anything from Fury about it. Find out which one of them Stark managed to seduce, cash out the bets, and put the whole thing to bed so we can all move past it." He turned on his heel and strode determinedly out of the room, not quite fast enough for them not to overhear him mutter under his breath.
Clint and Natasha stared at each other.
"Did he just say, 'I really hope it's Banner'?" Clint murmured uncertainly.
Natasha sighed. "He probably just doesn't want the memories of his childhood hero tarnished by Cap being with someone as...let's say 'morally flexible' as Tony. So how should we play this?"
Clint shrugged. "Come on, Tasha, you're the one with the interrogation skills, I'm just the triggerman." He flashed another cheeky grin, chuckling when she rolled her eyes.
"Well, I think our best bet is going to be staking out Stark's rooms; whoever he's getting groinal with, it's only a matter of time before the two of them end up there. We can't bug the rooms electronically, though, JARVIS would pick up on it and warn him. Think you're up for an old-school stakeout?"
Clint stretched lazily. "No problem. It's been a while since I gave the vents a dusting, not since we dropped that stinkbomb in the HR conference on synergy back in November, I think. I'll plot out an attack route that bypasses the security alarm if you get the gear together."
"Deal." Natasha sauntered towards the door. "Meet back here in an hour?"
"Make it two. And make sure you get a film camera, Tony's probably got a white noise bomb jamming any digital recording in his rooms." Clint's grin widened when Natasha turned to glare at him.
"не учи ученого, Clint. Don't teach a scholar."
And she stalked out of the room.
A/N: The thing Natasha says to Clint at the end is pronounced "ne učí učónogo" and literally translates to "don't teach a scholar"; it's a Russian idiom, equivalent to "don't teach your grandmother to suck eggs". It basically means don't give advice to someone more skilled/knowledgeable than you. (source)