Aside from the fact about urine being sterile, pretty much all of my argument comes from
common sense, pure and simple. Having a debate with these people who just go EWWWW GROSSIE GROSS!!

without thinking about it for two seconds made me think about it much more, and I realized a lot more. I still haven't gotten a reasonable counter-argument (from anybody, here or there) regarding all of this... common sense has a habit of owning people

People just go "showers are a place of cleanliness! nyahh!" and crap, when what does that mean at all? It's simply how you were raised, you're abiding a general teaching that really had no meaning in the first place. There's nothing actually dirtier or more immoral or anything about pissing in a goddamn shower. I'll bet more than half of the guys here who don't pee in the shower, probobly jerk in it. You think that's any better? It's not. It's not
bad either. You know why? All of it goes down the drain without a trace anyway. Dirt, mud, lint, your friggin' nut-sweat, this all falls off of you in a shower too. Guess what? It all goes down the damn drain. This isn't a matter of obsessing over wizz or something. I'm just thinking logically here, it's not complicated at all. If you honestly can't think of a better reason for not doing it beyond "Shower's are made for washing and washing alone!" or something completely erroneous like a line about swimming in your own pee, then you're just being neurotic about it.
Tassadar: a very good one, is what it is

It was just a joke at first but confirmation bias (surrounding yourself with people who agree with you until you think you're completely right) is definitely something that popped up around here. The fact that all the guys that did it thought everybody did it, and vica versa, is just an example of that.