Bucky gets fisted by HYDRA agents as a form of punishment/torture, and (whether they're purposely aiming for it or as a result of sex pollen - a la Blood from a Stone - or just as a result of the stimulation) orgasms as a result of it. Up to you whether this is a method HYDRA uses to break down pre-WS Bucky, to punish the Winter Soldier, or to punish/retraumatize a post-CATWS Bucky that had been regaining some of his memories/autonomy/sense-of-self etc etc.

Give me all the shame/humiliation of his body responding to it and getting off on it.

bonus points for:
++++ multiple orgasms
+++++ going from silent and stoic to having to force himself not to beg for it
++++++++ if someone else (preferably Steve and/or Sam) is forced to watch, and Bucky's added humiliation of them having to see him degraded this way and (in his view) enjoying it