Bucky learns/remembers that his partner -- and I am thrilled with this being Steve, but if someone could make this work with Sam/Bucky I'd love it -- has a massive, insatiable rape kink. Because of Trash (he subconsciously thinks he's kinda obligated? he wants to be everything his partner desires? this is what they used to do and he wants to live up to that?), Bucky takes the initiative to indulge Sam/Steve's fantasies by dirty-talking his rapes to him, maybe also while reenacting. The hitch is that this is actually horrifying and exhausting for Bucky, though he fakes enjoyment with the skill of long experience.

The other hitch is that while yes Sam/Steve has a rape kink, the fact that this all really happened to Bucky makes it thoroughly UNerotic. But he knows he should support Bucky through whatever strange coping mechanisms he needs, so he also grits his teeth and suffers through it.

This could be a long-term adventure or a sexual mishap that gets quickly resolved, but either way + the truth gets discovered when one of them can't take it anymore and freaks out