Same for people with foot fetish?
This is one of the most common fetishes out there. And it often goes with femdom. It's all oriented around subject feeling
submissive (like with any other femdom act), OR subject doing something that is considered "
dirty". Doing something that is considered "dirty" has a certain kink which people often enjoy experiencing. Like
@Exploizer explained, women walk with their feet, and if someone would to kiss/lick them, that would have been considered "dirty", and thus - that someone might experience the sensation of being submissive (especially if being ordered to do so, which adds extra bit of submissiveness), or that someone simply enjoys doing something that is considered taboo, or "dirty".
You've done the urination scene, so you probably understand that part, and it's not much different from foot fetish, subject did something that is considered "dirty", and showed how submissive he is. Same could be said for licking asshole. That is considered "dirty", but many people still love doing it, and they enjoy doing so.
And when I say "dirty", it doesn't necessarily mean that these parts need to be dirty for the subject to enjoy the act, I for one enjoy licking clean asshole, clean feet/toes, etc...
That said, if someone is into cuckolding, and cum cleanup (often goes with cuckolding), and foot fetish, then he'd probably enjoy cleaning cum covered feet with it's mouth/tongue, which is also considered "a dirty move".
Foot fetish is often the reason why chastity key (another femdom/cuckold play) is hanging on the women's anklet, you've probably seen the images, so if you were to worship women's feet, you'd be very close to the key, which would make your arousal even greater, because the key would be teasing you during your worshiping act.