Harry is sitting at home with his girlfriend, baby-sitting his little sister.
Once they put her to bed, Harry and his girlfriend start getting more comfortable, and just as Harry slides his hand up her skirt his mother walks in. "Harry, I want a word with you in the kitchen!"
So he follows his mother in and she says, "Harry, I never want to see you doing that again!"
"Why?" Harry asks.
"Because up between those legs is a black hole, and it has teeth in it. If you put your hand near it again, it will bite your fingers off!"
Harry promises his mother that he won't.
The years go on, and Harry finally marries his childhood sweetheart. On their honeymoon, Harry kisses his wife, then rolls over and starts to go to sleep.
His wife, rather annoyed, cries, "Harry, I've been waiting years for this day and I want you to make love to me!"
"Oh no, no, no," Harry answers, "my mother told me that up between your legs you have a black hole with teeth in it, and if I go near it, it will bite me!"
Laughing, his wife rolls over and picks up a flashlight; shining it between her legs, she says, "Can you see any teeth up there?"
Harry moves down the bed to look. "Well, Harry, can you?" his wife asks.
"No, there's no teeth, but I'm not surprised. Look at the state of your gums!"