Tony and Steve have never gotten along, but when Tony lies to his mom about bringing his boyfriend to his cousin's wedding, he needs someone to play the part. Lucky for him, Steve agrees, and the two might just get a little more than they anticipated out of it.
Tony and Steve don't get along, that's a known fact of the universe. However, when an alien invasion is pending and they have to convince them not to blow up the planet their only chance of survival is to allow the aliens to study Terran courting behavior. Too bad the only people around to do that are Steve and Tony themselves.
“Ah, tut tut, Thor All-Father; she no longer calls to you. She is open for the taking. We of power can feel it,” Proxima Midnight drawled, a smile tugging at her lips. This time when she hefted her trident she pointed it right at Thor, then proceeded to point it at each of them—Tony, Steve, Nat, James—and then back again to Thor. She held the trident steady, her muscles obviously strong and used to its weight in her hand. “Do any of you challenge me for her?” she demanded.
-or-
The one where Mjolnir gets repaired, Thanos' minions come knocking, Tony gives a speech, and there's a road trip. That may or may not lead to kisses. Oh, and Tony just might be worthy of lifting Thor's old hammer after all...
So, Captain America effectively manages to cockblock Tony for a year.
It's not Steve's fault. Well, actually, it is. But he was just proving a point - that if a superhero is gay, how can it be wrong? Steve just picked the wrong superhero to make the point with. Now America will think they're dating - and Tony's not going to be the guy to break Captain America's heart.
There's only one way out. To save face, Steve and Tony have to become fake boyfriends. Steve thinks the "boyfriends" bit will be the hardest to act... but maybe it's the "fake" part that will be the hardest act of all...
They loved each other, right? That was all that mattered. They loved each other; they made each other happy. There was no need to worry about the odd occurrences that kept happening like the phantom laughter Tony sometimes heard or the glass bowl Steve dropped and forgot about or the strange friendship between Wanda and Agnes... right?