A customer of mine told me this one.
A 10 year old kid walked in on his parents right when they were in the middle of having sex, mortified the kid runs out of the room slamming the door behind him. The wife looks over at her husband and says you should go check on little Tommy, he really looks upset.
So the husband gets dressed and goes to the boys room where he sees his son nailing his own granny, the father is mortified the kid turns to him and says not so funny when it's your mom is it!!!
It plays out much funnier when you tell the joke.