Sleepwalking

A woman's husband has a yearly conference. The first night he's away from home, their teenage son Tommy comes into their room at night and starts to make love to her, but she knows that it can be dangerous to wake a sleepwalker, so she doesn't say anything. He does this every night for two weeks and stops when his father comes home.

She realizes she's pregnant and has a baby boy.

The next year the same thing happens, she gets pregnant again, and has a baby girl.

The third year, she's feeling very guilty, and after thirteen nights of incredible passionate lovemaking she sits Tommy down and tells him, "Every time your father leaves town on business, you sleepwalk into my bedroom and make love to me. Bobby and Anna aren't just your brother and sister, you're their father!"

Tommy said "You think I was sleepwalking?"