The problem is that there's no set formula for something like this. Yeah, yeah, there's the question, the ring, the promises, but it's not like there's an empirical standard here. All pop culture agrees on is that each proposal should be unique, that it should occur at the perfect moment, but it's not like it's something Tony can code for, something for which he can ask Jarvis to calculate the statistical odds of success.
After weeks of fidgeting and fretting over it, he decides to go with a classic. He has the gold from his mother's engagement ring melted down and made into something he thinks Steve might feel more comfortable wearing, and books the Royal Suite at Le Meurice for a week. Tony flies them to Paris, figuring that a proposal at the top of the Eiffel Tower in the middle of the City of Light, it's a sure-fire winner. The two of them stand there at sunset—Steve looking out at the Seine, the white-and-grey stone of central Paris, the lines of his face softened with happiness; Tony nervously turning the ring box over and over in his jacket pocket, feeling grateful that he applied extra deodorant this morning.
Tony clears his throat, opens his mouth—and the evening sky lights up with explosions. "You cannot be serious," he says, because this, this is the time that what looks like a crack team of commandos decides to attack the Eiffel Tower? But apparently they are serious, and Tony's still Iron Man and Steve's still Captain America, so what the hell, they go to work.
Afterwards, panting, streaked with soot, Steve looks over at him and squints and says, "Did you get a tip or something? Good timing."
"Uh huh," Tony says, "yeah, something like that, if you'll excuse me I'll just be over here tearing my hair out in a delightfully Gallic manner." He waves away the waiter who's peering out hesitantly from behind a now half-melted strut—Tony'd booked out the entire Le Jules Verne restaurant, figuring that a perfect proposal should be followed with oysters and chocolate and artichokes and every aphrodisiac Tony could think of, before they tested the springs in their bed back at the hotel suite—but now that seems just a little pointless. Tony might not be entirely sure what a moment is, but he knows what it isn't.
After the Eiffel Tower incident—which, hey, at least now they both have the Légion d'honneur, so it's not like it was an entirely wasted trip—Tony decides that he needs a new approach. Simple, he decides. Minimalism is in, and Pepper reminds him that Steve tends not to like it when Tony goes overboard, so he decides that his next attempt will be made during a romantic picnic for two in Central Park.
Only he can't quite control his micromanaging tendencies, so he has the event catered and gets planning permission to put up a tent overlooking the Lake. It's all going pretty well—Tony even double-checked this time, and Steve is neither allergic nor averse to either strawberries or champagne—and then, just as Tony is deciding to inch a little closer to Steve, to make his move, the heavens open.
It pours. Not the romantic movie kind of pouring, either—no way Meg Ryan would have set foot in this. This rain is sheeting down, near to horizontal, churning the ground into mud and sending tourists running for cover. Naturally, Steve being Steve, he says, "Tony, we have to let these poor people in here. There's more than enough room, they'll get soaked."
Tony opens his mouth, closes it. Opens it again and says, "... sure."
"Thank god you had a tent set up!" Steve says, cheerful despite the weather, once said tent is full of a heaving mass of humanity, chattering and steaming as their clothes dry off. Someone has started eating the shrimp cocktail, exclaiming over the luxury of the Big Apple in a Midwestern accent; Tony’s trying not to think about it.
"Uh huh," Tony says. He's trapped next to an elementary school teacher from Poughkeepsie, who keeps trying to make small talk with him, while the ring box makes an awkward weight in his jacket pocket—he's not exactly thanking any deity just now.
The rain doesn't let off for a good half hour, which is more than enough time for half a dozen visitors from America's Heartland to recognise them. "Hey! Hey, aren't you Captain America? And Tony Stark?" It's the kind of observation that makes Steve bashful and Tony irritated, because come on now, either it's Captain America and Iron Man, or Steve Rogers and Tony Stark, that's not fair, and the next thing you know, Tony's in an argument with a retiree from Phoenix.
By three, Tony decides to give it up for a lost cause, walks back to the limo in squelching shoes. Steve is silent beside him, a little huffy that Tony would get snippy with poor Mrs Rosso, who was a very nice woman. When Happy sees them, he hurries to open the door for them, beaming and starting to say, "Congratu—"
"Ahaha," Tony says, aware that he's wide-eyed and soaking and sounds just a little manic, "Hogan, you're hilarious, but now is not the time for inside jokes! Because this is just an inside joke, a joke, Steve, time to go home."
Steve looks at him like maybe he's just hit his head or something, which is not an unfair conclusion, Tony has to admit—he gets concussions a lot, it's just extrapolating from past evidence.
Vegas, Tony decides. Normally, what happens in Vegas stays in Vegas, but just this once maybe the city will allow him to do something that sticks—impromptu but planned, he'll make it work. Only somehow, Thor overhears Tony suggesting the trip to Steve, and somehow, somehow, through the unpredictable alchemy of Asgardian enthusiasm, their weekend in Vegas gets turned into an Avengers vacation.
"Lo!" Thor booms when he catches sight of the Strip for the first time, "this place shines as strong a beacon as does my father's great dwelling! We shall make most merry here." And sure enough, everyone seems to have a great time—or almost everyone.
Steve hates Vegas. Tony's never seen him in such a terrible mood before, and certainly never heard him complain like this. Normally, Steve takes his surroundings with equanimity—Tony supposes a stint in the army will inure you to anything—but Steve wrinkles his nose at the Strip, disdains the velvet Elvis paintings, scorns the replica of the Sphinx. "But why," he says for the fifth time, "why would you build a replica of Venice in the desert in America?"
"You'd prefer actual Venice?" Tony says with a sigh. "Yeah, you’d probably prefer actual Venice."
Behind them, Thor scores big on the slot machines, punching the air in triumph, while Natasha—wearing something so slinky it's probably illegal—rakes in a pile of chips at the roulette table. If they keep going at this rate, the Avengers are going to be barred from every casino in Nevada.
Steve folds his arms. "When can we go back to New York?"
Tony spends most of the next two days in the bar at the Bellagio, getting comped for some really excellent whiskey while hating his life. Steve's holed up in their hotel room, working his way through a stack of paperbacks, and what the hell, what the hell, because he is trying, for once in his life, to do the actual noble and, and, right thing. Steve wants commitment, he wants marriage, he wants all this stuff, and Tony's trying to give it to him! And look, nothing.
"Honestly," he tells the bartender, "this'd all be so, so much easier if I could just code for it, you know? Have Jarvis test for bugs first, then'm good to go."
"Absolutely, sir," the bartender says, sliding Tony's glass away from him and replacing it with a cup of strong coffee.
They settle back into their routine in the Avengers mansion—you know, running a Fortune 500 company, saving the world on a regular basis, stopping Clint from shooting arrows off the top of the building in order to skewer the signs on top of taxi cabs, the usual—while Tony tries to come up with his next great plan. He gets nowhere fast, which is why three weeks pass and still nothing; which is why he's sitting in the living room at 11 on a Tuesday night, contemplating upgrades to the suit's biofeedback systems, while Steve works on a finishing off a charcoal sketch of Pepper for her birthday.
For a while, Tony's so absorbed in what he's inputting onto his tablet that he doesn't realise that Steve's humming to himself. Eventually, though, a line of the melody catches his attention and he stops, stares at Steve slack-jawed. "Are... are you humming a bastardised version of 'Hey Ya'?"
"Not sure," Steve says, carefully blurring a charcoal line with the tip of one finger. "Clint was playing it last night, it's sort of catchy." He continues humming it, weirdly slowed down and off-beat, looking content and at ease, and something in Tony's brain just sort of snaps.
"Goddamn it," he yells, throwing the tablet onto the coffee table, hearing the screen crack and not caring. "I just—I mean—would you just marry me, for the love of God?!"
"Uh," Steve says, looking up from his drawing. He licks his lips, looks as if he's considering his next action carefully, before he says, "... what?"
"I keep trying to do it well!" Tony snaps, "I've been trying really hard, but I've been forced to come to the conclusion that the universe hates me."
Steve shakes his head. "No, Tony, not that—the part where you proposed? Could we go back to that for a moment?"
Tony, undeterred, presses on, ticking things off on his fingers. "First there was Paris, because you know—Paris! It's Paris, that's totally how you convey the, the romance! And the things, and then the Eiffel Tower tried to blow us up, which is entirely unfair, I should still lodge a complaint with the French embassy. And then! Picnic, because they're simple and like, I don't know, honest, Pepper said it was good, I do not know because my track record argues against me knowing what the hell I'm doing, and the universe couldn't even throw me a bone, it threw me Mrs Rosso and I don't want to spend the rest of my life with her. And then third, third, you hated Vegas, of course you hated Vegas, I don't even know what the hell I was thinking, it's not like you even know who Elvis is."
Steve gapes at him. He looks, Tony thinks vaguely, not unlike a guppy—a slapped one at that. "You want to," Steve says slowly, after a long moment, enunciating each word carefully, "spend the rest of your life—"
"Well," Tony says, throwing his arms up in the air, "obviously, you don't keep trying to propose to someone because you're not stupid in love with them, Steve. But it never seems to work! There was a dog at the picnic! The tent smelled like wet dog, there is no romcom ever that's had someone propose successfully in a tent that smells of wet dog! And all I want to do is be with you, because I just, you're one of the best men I've ever met and for some unknown reason you've decided you want to wake up beside me every morning, and I think about you all the time, even when I'm with you. For you, I got to Brooklyn on a regular basis. For you I have voluntarily remembered a birthday, and I can get Pepper to sign an affidavit as to why that's a big deal. How am I supposed to adequately convey all of that if none of the fucking proposals work?!"
For some reason, while he's working his way through all of this, there's this smile growing on Steve's face—this huge, delighted curve. When Tony finally stops to draw breath, Steve stands, crosses the room in two strides, tugs Tony up off the couch to stand right in front of him, so close there's barely a couple of inches between them. He's beaming and saying, "Anthony Edward Stark, are you asking me to marry you? Right now?"
"Oh, god, the full name," Tony blurts out before he can help himself, then says, "I mean, yes. I mean, no. No, this is not how I'm doing this! There should be a band or, or flowers or something. I was thinking of hiring Tony Bennett to serenade you, Barry Manilow, you could pick, with hits from the Great American Songbook. Do you not want the Great American songbook?"
Steve's still grinning. "No, Tony. I don't want the Great American songbook."
Tony clears his throat. "... the Mediocre American Songbook? A one hit wonder? I should at least have the ring, I'm wearing pyjamas, there aren't pockets in pyjamas."
Steve puts one big, warm hand on Tony's shoulder, loops his other arm around Tony's waist. The smile on his face hasn't wavered, not once. He shakes him a little, very gently, and says, "Tony. Look at me."
Tony does, and then takes a deep breath and sighs. "Yeah, yeah, I'm asking you to marry me. Right here in this living room in pyjama pants with no band or aphrodisiacs or skyline or anything."
He's barely finished speaking before Steve hisses "Yes" and kisses him—cups his face in his hands and kisses him, bending him backward so that Tony has to cling to him a little—and then, afterwards, a lot.
A couple of hours later, Tony lifts his head off the couch cushion—feeling very satisfied—and says, "If I'd given myself just one more run at it, it would have been epic. Epic, Steve."
Steve grins, the grin that Tony knows by now means, "I'm humouring you because you're ridiculous and it's endearing," and says "I know, Tony. I know."
They get married seven months later. Rhodey is Tony's best man, and of course Bucky is Steve's. Agent Coulson officiates, a long-suffering look on his face, but Tony's pretty sure that Coulson gets it on with the Lady Sif at the reception so it can't have been all that torturous for him. (When Tony sees him for the first time after the honeymoon, he tries to fistbump him, because come on, hot alien Viking warrior princess lady? Who wouldn't be proud of a guy getting with that? But Coulson just arches an eyebrow at him and makes a bland remark about never kissing and telling. Spoilsport.) Clint cries a little during the ceremony, but threatens to shoot anyone who brings it up ever again. Natasha just smirks at him regardless.
Pepper spends the first half of the ceremony on the edge of her seat, waiting for Tony to freak out, and the second half grinning because he's clearly not going to.
(Though when she and Natasha get married a couple of years later, she has to tell the videographer to leave out several key bits from the speech Tony gives at the reception, because good god. Steve spends the entire speech mouthing apologies to her, because marriage has been all kinds of good to Tony but in fundamentals—oh, in some things he's never going to change.)