Peeing in the shower is gross. However, peeing in the shower will not harm anyone or anything. Thus, one cannot take a stance on whether peeing in the shower is acceptable until one comes to terms with this contradiction.
This is a rare case where both sides of an argument are correct, and yet there is still a single, unassailable conclusion that is available to everybody. Here it is: You may pee in the shower so long as you don’t admit to it.
Why it is not okay to talk about peeing in the shower
Because it is gross. It’s pee. It came out of your body, and people don’t want to hear about it going anywhere except the toilet. In fact, they probably don’t want you to regale them with stories of your pee going in the toilet either. Just keep it to yourself.
In the rare case that you know your conversation partner is okay with shower pee, there is still no reason to discuss it. You both do this gross thing, and you don’t think any less of each other as a result. That is settled. You can move on to discussing something more pleasant now.
Why it is okay to pee in the shower (in secret)
Because it’s just urine. It’s not like the acid blood from Alien, for pete’s sake. If it would happen to touch your skin, it will be instantly washed off by the shower you are standing in.
The same goes for your plumbing and your shower floor. The moment after the pee touches them, it is immediately washed away. We don’t need to discuss its color or acidity or any other properties. It will not be in contact with any surface long enough to have an effect on it.
Is urine sterile? It does not matter! But to answer the question, it may have some bacteria in it. It’s not full of germs in the same way that poop is (poop is largely made of germs). It’s not somebody else’s coronavirus-laden cough. It is urine, it came out of your body, and it will soon be streaming down the shower drain.
If you have open wounds on your feet, won’t the pee momentarily contact them? Well yes, but if you have open wounds on your feet, your feet will contact other gross and germy things like the floor so you should cover up your open wounds with waterproof band-aids if you’re so afraid of contamination. (This might make sense for surgical incisions; I do not think it is a real concern for ordinary minor scrapes.)
How to navigate this question socially
Peeing in the shower is still a personal choice. You are not required to do it (even though it might save a bit of water and time). If you prefer not to pee in the shower, you do not need to change anything about your life, except that you must stop asking your friends whether they pee in the shower. If they volunteer this information, do not engage.
For the rest of us: Don’t bring the subject up. If directly questioned, you can simply answer that peeing in the shower is gross. This is a slight dodge, but it is 100% accurate.
Do not pee in a public shower while others are present.
And always, always, always ensure that the floor and other surfaces have been rinsed before you get out of the shower. Leave no trace.