Look, it's not just about peeing.

I mean, yeah, there's some erotic thrill there but it's got so much subtlety to it.

Firstly, there's the whole desperation aspect. Socially we aren't allowed to relieve ourselves openly so we have to go somewhere discreet to do so. This leads to two scenarios, both hot: a) having to go somewhere undignified and possibly lacking privacy (this will be addressed in further detail) or b) desperately searching for a proper place to empty themselves, which has a tension aspect attached to it.

So, understandably, with such a scenario there is the possibility of failure. Failing to find a suitable place to go results in a loss of control somewhere in the open. Public embarrassment hits an empathetic level that draws you closer to the subject. Wanting to comfort someone can be arousing.

On the other hand, success can lead to relief. The feeling of not having to hold back any longer, especially after being right at the edge of bursting. That long, satisfying sense of release that accompanies reaching their destination without losing control.

And finally, the earlier point of privacy. It's assumed that we're all owed a modicum of privacy when using the toilet so to disrupt that would be unpredictable, unwelcome, and awkward. There's a moment of panic, distress, and flustered indignance of being caught in the act. Perhaps this hits a rape-ish appeal but it's still a turn-on.