Granted we all have to urinate, but to some it is a fetish they wish to pursue. There are sites that feature girls peeing in public, and others wish to see girls and guys drinking pee gushing out from others. This kind of porn is usually labeled watersports. At times it is a BDSM act forced onto another, but sometimes it is an act that is voluntarily practiced by the participants. Some just like to see someone peed upon, while the use of hidden bathroom cameras to watch someone take a piss is also common.

Learn more about watersports in Wikipedia.