It's not like anyone's incapable of peeing first or holding it, they're just comfortable enough not to care.
Urine is pretty damn sterile already (even less dangerous if it's your OWN), so the only real issue with it is the ick factor. In the shower, it's not hard at all to avoid getting any on yourself in the first place, but even beyond that, if you do, you can just wash it off because you're IN THE SHOWER.
The people talking about it sticking to things must either:
(a) shower without water (DRIED urine can be a *****)
or
(b) pee in their showers with such volume and regularity that their woefully infrequent and inadequate bathroom-cleaning can't take care of the issue.
I'm not even sure HOW long it would take for sediment to become noticeable, so I'm skeptical that (b) could ever happen in a typical period of habitation. That said, you can clean damn near anything out of a shower since it can withstand some seriously frightening chemicals. When I was younger, I used to help my father with his janitorial service and let me tell you, urine is EASY.