I had friends who seemed to be into it. When I was 11, I ran around with a kid who was a little older than me. One day we were using a public bathroom and he told me he liked to let a little pee go in his underwear because it felt good.
Later when I was in high school, my best friend's younger brother by one year was washing his underwear out in a bucket in the garage. I asked my friend about it and he said it was a private thing but I think younger brother was into peeing or messing his underwear and his mom was not happy, thus making him wash his own underwear.