Is it normal for people to have sex in hostels?

I like travelling on my own and I generally sleep in cheap youth hostels where you have to share a dorm with a bunch of other people. A couple of weeks ago I was staying in a 32-people(!) dorm in Amsterdam and I am quite sure I could hear a couple having sex in one of the beds in the room, and it was not even that late at night.

It did not bother me too much(mostly because I was not trying to sleep at the time, and I couldn't see anything), but I was wondering how common/acceptable this is.