Not the author, but your comment popped into my mind today, and I realized I really like that what he's afraid of is being abandoned. Bucky's damaged enough to think that Steve and Sam expect full sexual compliance from him, and he fears punishment if he fails to perform. But my reading of it is that he isn't so broken that he believes they'd torture him on the scale Hydra did -- and even as difficult as it is for him to resist the will of others, he might not let them get away with it if they tried. Abandonment, on the other hand, is a much more realistic terror. He fails to please them, they leave. I don't know why abandonment is the worst (he needs love so badly? he'd be so vulnerable on his own?) but it's really effective as a choice of motivation.