Bucky as the WS has a severely limited understanding of consent. He's never been given the opportunity to consent or not. He doesn't know it exists. He's not a blindly sexual being either - he understands hurting people, he understands (on some level) humiliation), but. He really does not understand how all this works.

He's locked in a room with Steve, towards whom he feels that strange cocktail of emotions and memories, and he's been dosed with an absolutely absurdly potent aphrodisiac.

What happens next is up to you.