Maybe Bucky got his dick badly injured on a mission, or maybe Hydra is just that twisted, but for some reason he got both cock and balls completely removed while he was the winter soldier. he can still feel pleasure (in his ass and around his urethra) and he can still come (wikipedia says like 30% of ejaculate is produced in the prostate, which he would still have).

I want to see Steve's utter shock (and shameboners of course) at what Bucky's lost (but no feminization please).

+because of mind wipes, Bucky doesn't remember ever having a dick

++they try to have sex anyway and Steve ends up coming like four times before Bucky can, because he's missing so many nerve endings (and Steve is that turned on)

++++++they end up sticking fingers and other objects in Bucky's urethra to make him feel good