HYDRA has fried the Asset's brain circuitry so much over the years that they notice/realize he can only get an erection and ejaculate when he is in the chair or is being zapped with a stun baton (at first it's just on the body, then they overdo it and have to resort to more direct application to the groin). Ejaculation and orgasm is ALWAYS excruciatingly painful. This can be about trash parties employing electric current (have some fun! maybe some adventurous folks have another person zap the Asset while they are fucking him and get a little bit through their own bodies) and/or aftermath trash where Steve and a recovering Bucky try to deal with this new reality (again, maybe Steve can get zapped a little, maybe they learn to enjoy the pain).