Until you’ve swum in a public pool that requires full body wash and shower, naked, you don’t realise the difference it makes to the chlorine amount required to keep the pool clean. Denmark has explicit instructions about where to wash, pre-soaped single use sponges that must be used, but hardly any chlorine in the pool water.

Humans are filthy, yo. I’d rather everyone cleans properly before I have share pool water with you.

So when kids pee in the pool all the pee stays inside their pants I'm sure...

I know it was absolutely a thing in the past, my parents talked about it for example, but I've never seen all-gender changing rooms in a French swimming pool. Though I've seen the obvious signs of the retrofit in a few places, where you can clearly see that it used to be one big room that got separated in two.

I think you're right that they don't trust us to properly shower before going into the pool though, because to this day, I still see like ~40% of people barely showering enough to rinse their sweat off. A lot more people are actually showering than when I was a kid though (myself included)

Edit : actually, I've been in some pools that were constructed more recently that have mixed changing rooms, but these have individual stalls and the showers are still gendered

TLDR : the showers aren't all-genders as far as I know. The "changing rooms" sometimes are but in this case it's just a big room with stalls and lockers so privacy isn't really an issue.

you don't shower in France normally

FTFY

....here in the states its like a before and after thing. We are allowed to take two showers lol