Best Math Joke of All Time

In the summer of 1975, Prof. W. O. J. Moser (McGill) hired me to organize the papers left by his recently deceased brother, Leo Moser (University of Alberta). There I found these notes of an exchange between Leo and Professor Joachim (Jim) Lambek (McGill).

Leo:   Hey, Jim, look at what I can prove: The number of people who have had intercourse an odd number of times is even.

Jim:   You can’t prove that.

Leo Moser starts to give the proof.

Jim:   Oh, go fuck yourself.