Do you wanna watch me? - Well at that point it might just be rude of her "not-girlfriend" to refuse.
Now I'm not one to pass excessive judgement on anyone, but I just don't see the appeal of watching your crush pee.
A fetish can manifest for any of a huge number of different reasons. If I had to guess one of them, intimacy involves vulnerability, and there is an aspect of vulnerability to doing one's business.
This. Humans are wild and kinky creatures. There's a lot of things we probably shouldn't find sexual, like breasts are the way a mother feeds her baby, but they're a commonly sexualized part that most people don't think twice about. That's a vanilla example, but trust me when I say peeing is one of the lesser weird fetishes out there haha