The dome pool is also the one part where you are required to wear a swimsuit. I'm pretty sure the reason is that if you didn't, let's put it like this, that water would become a health hazard pretty quick.
Swimsuit or not, people can behave inappropriately. Let me put it this way, the couple were having some form of sex and thought they were being discreet. The acoustics betrayed them.
I mean I don't doubt that it happens, but the swimsuit rule probably cuts down on it. And to be fair, that pool is possibly the sexiest place in Berlin.