Author's Note- In an attempt to branch out on my pairings, I came up with this.
"You're a laboratory experiment, Rogers. Everything special about you came out of a bottle."
1. It's a jibe only Tony can give out. Snarky, self-confident, and just a tad hypocritical. At first, Steve tries to ignore it, because it's coming from a man who wouldn't be a hero if he wasn't a genius mechanical engineer. He'll try and push it out of his mind, that it's just a little bit true, that when he gets depressed or frustrated, he'll think, What would I be without the experiment? A punching bag, that's it.
2. It doesn't occur to him until much, much later that laboratory experiments and bottles are some of Tony's favorite things. It doesn't occur to him what the words really mean until he stops seeing the suit of armor and just starts seeing Tony.
3. Tony actually likes him, all of him, which never ceases to amaze Steve. Maybe it's because Tony's only ever known him as tall and broad and strong. Even with all the screaming Captain American fangirls and the merchandise and the media analysis, he's not quite comfortable like this. To him, his body is an alien and every time he looks in the mirror, he still expects to be short and skinny. Just because he can act like he owns it when he's with the Howling Commandos or the other Avengers doesn't mean that he's actually Captain America, hero. In his head, he's still Steve Rogers, Brooklyn orphan. Nobody.
4. Nobody, not Pepper, not Rhodey, not anyone, knows how absolutely fascinating Tony finds Steve. They (they being everyone who isn't Tony, and sometimes that even includes Steve) think it's just normal infatuation, normal affection. It's not.
5. "Not to ruin the moment," Steve gasps, as Tony does something with his hands that should not feel that good but does, "But you could have anyone. Anyone, Tony."
6. Tony just rolls his eyes, because yeah, he could have anyone he wanted, but right now, the only person he wants is the man lying next underneath him, all sleek blond hair and chiseled frame. And because maybe Steve doesn't get it, but it's not so much the fact that the Serum made him so incredibly strong and attractive (not that that hurts, not at all) but that Steve is literally human engineering.
7. Engineering the human body has never been as exciting to Tony as it is now. At first, he reads Steve's file, just to see how the Serum worked. That turns into digging into Howard's old files (and damn, Tony never thought he'd want to do that) and reading them through, each and every word. Finally, he decides that diagrams and journals can only do so much, and he goes looking for the real thing.
8. "Thing is," and now Steve's words are broken up by Tony kissing him, distracting enough to almost make him forget what he wants to say, "The thing is, maybe you were right that day on the Helicarrier. Maybe I am only a laboratory experiment." He'd never admit it to anyone else, but that self-doubt is always there, because has he really done anything to deserve being a hero? No. Just because Peggy picked him doesn't mean that Howard and Erskine and all the others couldn't have made anyone else into a hero. It doesn't mean Steve Rogers is anything, really.
9. "Really, Captain?" And there's Tony's breath in his ear, warm, legs twisted together. "Then it's a good thing that I've always enjoyed laboratory experiments."
"You're a laboratory experiment, Rogers. Everything special about you came out of a bottle."
10. And finally Steve gets it.