They tell the doctor they haven't been able to have sex in years. The doctor says he'll examine them both to find out what the problem is.
First he examines the husband, and after a few minutes the doctor writes some notes and then says, "OK, tell your wife to come in." So the husband leaves and tells his wife to go see the doctor.
The doctor tells the wife to take off her clothes and stand in the middle of the room. She does as she is told and then the doctor walks around the wife a few times, makes some notes and then tells her to put her clothes back on and send her husband back in.
When the husband enters the room, he asks, "So, doctor, do you know the reason why I can't get an erection with my wife?"
The doctor says, "Yes, and there's nothing wrong with you. I looked at your wife naked and I couldn't get an erection either."