Your pee is actually sterile to your body so urinating does clean a genital piercing. In fact, when I had mine done I asked how to clean it and the piercists shrugged his shoulders and said "go pee".
Oh I didn't say it WOULDN'T clean it but instead of planning to drink lots of extra water so that you pee and it cleans it after sex it seems like one would just clean it with soap and water because to clean it relatively soon after sex by peeing one would have to be in a almost needing to pee state during sex and that ain't cool. And if you started drinking all the water AFTER sex it'd be all crusty waiting to be cleaned likely.