The doctor agrees to examine him and begins by showing him various drawings.
First the doctor draws a square and asks the man to identify it. The man immediately says "sex".
Next the doctor draws a circle, which the man again identifies as sex.
Thirdly, the doctor draws a triangle, which of course the patient identifies as "sex". The doctor puts the drawings away and says to the patient, "Yes, I do believe thhat you have an obsession with sex."
To which the man replies, "I'm not the one with the obsession! YOU'RE the one drawing all the dirty pictures!"