I've never heard my parents having sex, although that is mostly because not long after I was born they stopped even sleeping in the same room.
However, I was witness to a friend of mine after he walked in on his parents. A group of us had been out to dinner and we went back to his house afterwards to watch some tv, and when we got there he went to let them know he brought company over and they were having sex. Didn't even notice him open the door, actually. He shut the door, ran back to the living room, and sat there in shock for a while. We were all around 16 at the time. It was funny for the rest of us, but he was a bit disturbed. So I guess it depends on your son.