Found the actual joke in Asimov's The Jokester at http://www.ippt.gov.pl/~vkoval/fantasy.html http://www.ippt.gov.pl/~vkoval/fantasy.html: "Johnson," he said,

Found the actual joke in Asimov's The Jokester at http://www.ippt.gov.pl/~vkoval/fantasy.html http://www.ippt.gov.pl/~vkoval/fantasy.html: "Johnson," he said, "came home unexpectedly from a business trip to find his wife in the arms of his best friend. He staggered back and said, 'Max! I'm married to the lady so I have to. But why you?'"