You're in (urine, hehe) in the shower to clean yourself. Then hopefully you soap and scrub all your parts including your feet with hot water, so if any urine gets on your feet, they'll be cleaned anyway. As long as you're not pissing on the wall, it'll all get washed down the drain. Secondly, urine is sterile, and while it can be a skin irritant, it can still even be drank for rehydration. It's not a disgusting substance and while generally I don't want people to piss on me, if it's done in a shower, washed down a drain, and the body is washed, I see no reasonable explanation why it's disgusting to urinate in a shower. It saves quite a bit of water as opposed to getting out while the shower is running, flushing the toilet which uses several gallons of water per flush, and then hopping back in.