Originally Posted by pititom

I have been in hotel's wellness areas where I was politely pointed the "Nur textilefrei" sign by locals who seemed disturbed that I would keep my swimtrunks.

As for enforcement, in a recent trip to Munich, I wanted to try the Erding therms. There, I was asked by the staff to remove my swimtrunk : trying to say in my best German, that I would rather keep it, I was clearly answered that I would be evicted out of the complex if I didn't get naked.

So, yes...enforced. But that is not my question (after all, each society enforces its own regulation, like it or not) but more where is the origin of this and the reason too.