I'm not sure if this has been posted before.
A couple enter a doctor's office.
"Doctor, please help us. We are having trouble having sex", the man says. To which the doctor replies "Off course. How can I help ? What is wrong ?".
The woman says "Doctor, we aren't sure but something is wrong. Can you please watch us and tell us what's wrong ?". The doctor is startled by the request, but is very curious so assents to their request.
So the couple enter a room at the practice, undress, start having sex in front of the doctor. At the end, the woman asks "Doctor, doctor… did you find out what is wrong with us ?" The doctor, still a bit in shock shakes his head and says "No, no! Everything went fine. There doesn't seem to be anything wrong." The man says "Alright then. I hope you are right doctor. How much do we owe you?". The doctor says "50 dollars". So the man pays and the couple leaves.
A week later, the couple comes back to the doctor's practice. "Doctor, it is happening again. Something is wrong. Please help us.", the man pleads. The doctor nods his head and lets them in a room. The couple undress in front of him and make passionate love in his office. It takes about thirty minutes for the whole thing to be over and the doctor says "Look, there really is nothing wrong from my point of view." The woman looks at him "Doctor, how can you be so sure ? It still doesn't feel right!". The man pays him the 50 bucks and they leave.
Another week passes and the couple is back. The doctor sighs and just lets them into the room. They undress, and work at it in a few different positions in front of the doctor. At the end of the ordeal, the doctor unhappy about the couple coming back all the time says in a loud voice "Look! There is nothing wrong with you. Will you please tell me why you keep coming back here?!". The man, who was counting 50 dollars in his wallet says "Doctor, it's actually quite simple. We just want to have sex, but renting a room at the hotel costs 150 bucks a night, and you only charge 50 dollars." The man pays the doctor and the couple leave.