A little boy is with his mother while she is dress shopping. He wanders off and after much looking she finds him under a mannequin's skirt looking up and staring. She is concerned that he is taking such an interest at a young age and tries to put him off by telling him that he needs to be very careful about that part of a lady's anatomy because they have "Big teeth, sharp teeth, sharp teeth that bite". She repeats it to him a few times and he grows up genuinely believing that women have teeth there, big teeth, sharp teeth, sharp teeth that bite.
He grows up and gets his first girlfriend and things are getting hot and heavy on the couch and despite all of her urgings, he won't go anywhere near her ladybits. She says "Why won't you touch me down there?" He replies, "Cause you've got teeth down there, big teeth, sharp teeth, sharp teeth that bite."
"What?"
"You have big teeth, sharp teeth, sharp teeth that bite."
"That is the most ridiculous thing that I have ever heard..Look I'll show you"
So the girl lifts up her skirt and pulls down her panties and pulls her lips apart. "See no teeth!"
And he says "Well, with the state your gums are in I am hardly surprised"