Aah the memories.. had it back in Ireland when we just moved in.
The guy nextdoor was very loud with his girlfriend.. after the second night of " oh Gavin,yes Gavin,not in there Gavin..etc " i met him in the hallway the next morning on my way to work.
While closing my door i said " morning Gavin,how are you"..the guy stood there looking baffled , i laughed and walked on.. never again did Gavin and his missus go loud in the bedroom
We had that issue in our last apartment. I took it like a man for a while, but at some point it was too much and I called him at 2:30 a.m. (after he had finished). He told me to frak off. Next morning, we noticed that the woman who left from his apartment wasn't his wife - he was cheating on his wife while she was on a business trip.
Anyway - after that incident, we started to cheer loudly every time they were having sex upstairs. That worked like a charm. After the third time or so, the noise stopped completely.
I neither live with my mother, nor do I have kids, my neighbors also don't have any kids, so I don't have to worry about either making my mother blush or "corrupting" youngins. Therefore, I will be as noisy as I want.
I have read (can not find source now, I did look though!) that women are better able to enjoy sex if they feel free to be noisy. More enjoyable sex also means they're more likely to become impregnated (during orgasm, internal muscles work to help draw sperm in).
So, ask them to be considerate if it is disturbing your sleep, otherwise, mind your own business.
Post a copy of the Hausordnung on their door with the appropriate section about quiet hours highlighted in bright yellow, and ask that they conclude their love making promptly before 21:59:59.
Reminds me of a neighbour on the floor above me when I lived in a tower block in London 20 years ago. I never met her or knew her name, but I knew that her boyfriend's name was Joe and what his favourite.....err....docking station was.