Hydra trained Bucky early on to come on command, and they always made sure he came at least once during trash parties, no matter how vicious they got. Once he is free and has recovered enough to try sex with someone he actually loves (I'd prefer Steve or Sam, though anyone is fine except Nat or Tony - Bucky/Nat is my friendship OTP and romantic NoTP, and I just don't really like Tony). Unfortunately, every time he tries to have sex, even if he's not getting flashbacks or overtly freaking out, he still ends up too anxious and caught up in his head to come; sometimes he can't even manage to get hard, or gets hard for a few minutes only to go soft again a few minutes later for no reason he can figure out. This, of course, leads to all kinds of shame and internal victim blaming, as he wonders if Hydra made him so sick and corrupted that he can only get off on torture from evil people.

Bonus for:
-Bucky never told his partner that he was raped, and desperately wants to keep hiding it.
-Bucky starts hiding from his partner and refusing sex, leading to his partner worrying that they've done something wrong.
-Bucky trying to masturbate to thoughts of what Hydra did to him and getting off.
-Bucky suggesting "kinky roleplay" where his partner acts out some of the things Hydra did to him (without letting on that it's actual things that were done to him), and still doesn't get off.
-Whatever other terrible things you can come up with that involve Bucky being unable to get off with the person he loves even though Hydra always made him come.