Cause Bucky's memory is screwed up because of all the brainwashing, and the way he's damaged forces Steve to either hurt him by rejecting him or do something terrible by lying to him for an extended period of time. If they actually have any sex, it'll be very dubcon since Bucky can't remember that Steve wouldn't've consented, and Steve would be consenting for all the wrong reasons.
Re: Steve never loved Bucky