So Steve may have some repression issues. Other people's business is their business and he's not gonna judge it, but he's always been very careful not to let the way he looks, feels, or thinks about men anywhere near even the abstract concept of homosexuality. Even Bucky. Especially Bucky. It's something he outright avoids thinking or learning more about, because he doesn't want to know what will happen if he connects those dots.

Until he's chained up in some baddie's dungeon getting smacked around by the guards, and it turns sexual. And suddenly the gritty details of sex between men are something he can't not think about, not if he wants to get through this.

Basically give me Steve Rogers having his Great Bisexual Epiphany right after he's been violently, traumatically raped. Anguish, disgust, shame, confusion, despair, and a couple of decades' worth of pent-up realization, all smacking him upside the head at the same time, on top of all the ways this experience is fucking him up already.

(Bonus if, to the best of his knowledge, Bucky hasn't endured similar treatment, because Hydra brass would never let the rank and file mishandle a prized weapon like that. Captured enemies, on the other hand, are a perfect target for gratuitous humiliation and abuse. And bonus if that makes Steve ten times more determined never to let Bucky guess what happened or what Steve's feelings for him are.)