I remember when I was in Elementary school (I sexually matured early and was a very horny boy) I got so horny while sitting in class, my dick was as hard a rock, all I did is rub it a few times and I had an orgasm, shooting a huge load of cum all over the front of my jeans... it was awesome! A girl sitting next to me saw the whole thing. True story And yes... sometimes I do pee a little in my underwear and I like it.
I cant understand it, but judging by the number of videos of guys (and a few girls) pissing their pants, along with the number of views and comments, it most be about the most popular fetish on the planet.