The winter soldier wasn't supposed to be having sex so they rewired him to just fall asleep if things started moving in that direction. Some of the guys at hydra use this to their advantage.

Later, Bucky doesn't remember that aspect of his time at Hydra at all and it was completely missed during his deprogramming so when he tries to start a sexual relationship with someone he keeps passing out instead.

Bonus
Bucky has dreams of being raped while sleeping during this time.