So Bucky is now with Steve after Hydra fell, they're fumbling along into a relationship, and then Bucky gets Steve into bed. Bucky dirty talks like a pro, low velvety voice whispering filthy things into Steve's ear about how he loves Steve's slutty hole and his perky titties and how desperate he is to get fucked. And Steve loves it, rides that edge of helplessness and shame and comes like a speeding train
EXCEPT
Bucky doesn't realise that this is kinky behaviour and thinks this is how normal vanilla sex is supposed to go. Because when he was getting raped by hydra lots of them liked to verbally degrade him and make him feel humiliated and awful while also making him feel good and come (all the more to humiliate him and prove them right about being a slut for it). They probably did terrible things to him too, all kinds of painful torture, but those were the only time he ever got to feel good, and he wants to make Steve feel good.
And when Bucky fucks Steve he's triggered to all hells but he wants to please Steve so even though it's triggering him he keeps going. And he deliberately tried to recall the worst most nasty things they said to him because he wants to be better for Steve. So basically Bucky us all dirty talking Dom but on the inside he is constantly on the verge of a panic attack. And also he had trouble keeping it up sometimes because he doesn't want to hurt Steve like this, even if Steve wants it, he knows this isn't right, but he doesn't know any other way of having sex that includes making it good for the bottom, and he really wants Steve to feel good.
Cue self loathing and confusion
Bonus if Steve finds out but only after weeks of mindblowing orgasms and then he is promptly horrified and then cue more angst and self loathing.