Peter walked in from work one day and his wife was sitting on the sofa with his girlfriend.
While trying not to look surprised he said, “What’s going on?”
“You tell me?” replied his wife.
Pete said, “I don’t know, you’re sitting on the sofa with a stranger. “
“A stranger, hey?” shouted his girlfriend, “I’m not a stranger. We’ve been having sex for six months!”
Peter looked at his wife and said, “Is this true?”
This joke is included in the Black Book of Jokes. You can order it from here.