To answer your question, I recommend that you definitely use a disposable diaper not a pull up under wear style. Even though you are a thin man, I believe "small" must snitch up nice and tight around your waist, leaving you plenty of room in the pouch area to get an erection. As us men know, that is one of the plus points of wearing a diaper to bed or even during the day, we can masturbate and enjoy it knowing we have full protection. Don't forget, masturbation keeps your prostate healthy, so use those diapers .