A concerned patient asked the doctor if masturbation is harmful. "Not usually," answered the doctor. "Not unless you do it too often."
"How about three times a day?" the patient asked.
"That seems a little excessive. Why don't you get a girlfriend?"
"Oh,... I already have a girlfriend," the patient replied.
"I mean a girl you can live with and have sex with?" asked the doctor.
The patient said, "I've got one just like that!"
So the doctor asked, "Then why do you masturbate three times a day?"
"Because... she won't have sex during mealtimes!"