The Winter Soldier (under orders, or confused as his programming breaks down, or Bucky acting under a trigger phrase à la 'sputnik') rapes Steve. It's brutal and painful, all the more so because they both wanted each other pre-fall but neither of them ever made a move. Later, after he's broken free of his programming and recovered his memories, Bucky avoids Steve out of shame, but Steve finds him and makes it clear that he forgives him. They strike up a reckless, ill-advised sexual relationship, which Steve probably initiates... and sooner rather than later, Steve has to nope out while they're fucking. Maybe not the first time, but the second or the third. He's furious with himself for letting it affect him, Bucky's in a guilt spiral and trying to give Steve space--so Steve, who under normal circumstances would rather have a tooth pulled than seek out comfort when he's hurting, basically tells Bucky to just shut the fuck up and hold him for a while.

Bonus if Steve nopes out due to rage, disgust, or a barely-controllable urge to struggle and fight back--something besides pure undiluted distress. Double bonus if he confesses the urge to fight back to Bucky, Bucky goes "sounds legit, go ahead and hit me," and they have fucked-up sad-angry semi-roleplay fightsex.