The Winter Soldier was always catheterized - they had to make sure his bladder was empty before cryo, it made things simpler for medical procedures, and it prevented distractions on missions. After years of that he's lost voluntary control of his urinary sphincter. So as part of his recovery, Bucky has to be re-potty-trained from the start. Bonus if whoever is helping him struggles with finding it hot, double bonus if he also got used to enemas+plugs instead of controlling his own bowels.