I think it’s because you feel inhibited, so your brain manifested it physically. My guess is that you are a sensitive person that has a fixation on being the most civil person you can be. So the idea of peeing next to another person just doesn’t feel civil to you. Or at the very least feels uncomfortable. So I think you have to learn to think in a more spontaneous, wild way. Not too far like an animal lol. But learn to let your inhibition/guard down a little. I struggle with these types of things too, that are related to stuttering.