Well, I can remember after I had been masturbating for a while I discovered it felt good to lean back first and pee on myself, so pee landed on my stomach and ran back down my cock and over my balls. Then I would wank and my cock would be wet and delightfully slippery. I still do this sometimes, lying in a bath although when I was young I did it sitting on a toilet.
I'm not sure when I first thought about girls peeing and got turned on by the idea, but it's been a big fantasy for years. I had a rather straight girlfriend at one time who once fondled my cock while she was sitting on a toi