Steve, on a mission for SHIELD, unexpectedly barges in on some covert Hydra skullduggery. It... doesn't go well for him, in the usual deliciously awful HTP way. And all the assailants are in masks or full-face helmets, so he doesn't get a good look at them.

Afterwards, when he's bottling the whole thing up and not telling anyone what they did to him, he finds himself hanging out more and more with Rumlow. The easy camraderie, the mutual competence kink, the lack of questions asked--it's all kind of comforting. They end up becoming fuckbuddies. Rumlow's unfazed when Steve occasionally flips out and goes all fight-or-flight during sex; says he figures they've both seen some fucked-up shit.

Of course, what he doesn't tell Steve is that he knows exactly what's got Steve so jumpy, since he was one of the rapists.

Bonus if Rumlow actually does like Steve, even admires how stoically he holds up in the aftermath... it's just that Rumlow also has no qualms about being the most brazen of douchebags, and after he first gets a taste, he finds himself wanting to get Steve alone and fuck him six ways from Sunday without any teammates getting in the way.