Oh, it happens, and the parents of our so's know that people have sex.
My wife's mom has heard us more than once.
We stayed at a cabin with my wife's entire family. My wife and I took a shower while her mom was in the cabin. Her mom saw us go into the shower together. She knew what was going on.
My wife and I screwed in a hotel room that we were sharing with her mom while her mom was around the corner getting ready for bed. (We thought we were sooooooooo sneaky, but looking back we were just young and dumb.)
My wife and I even had sex while she was talking to her mom on the phone, although most of the conversation on my wife's end made heavy use of the mute button on her phone!