"Keeping Score" joke
A couple has a male friend visiting from out-of-state, when an unexpected blizzard blows in and keeps him from traveling. Since the couple doesn't have a guest room, the friend says he'll find a nearby hotel and be on his way in the morning.
"Nonsense," says the wife. "Our bed is plenty big enough for all three of us, and we are all friends here." The husband agrees, and before long they're settled in: Husband in the middle, wife on his left, friend on his right.
After a while, the husband begins snoring. The wife sneaks over to the friend's side of the bed and invites him to have sex with her. Naturally he'd like to, but he's reluctant. "We're in the same bed with your husband! He'll wake up, and he'll kill me," he says.
"Don't worry about it," she says, "he's such a sound sleeper, he'll never notice. If you don't believe me, just yank a hair out of his butt. He won't even wake up."
The friend does and, sure enough, she's right. Her husband sleeps right through having a hair yanked from his butt. So, she and the friend have sex, and then she goes back to her side of the bed.
After about twenty minutes, she's back on his side of the bed asking him to do it again. The same argument follows, another hair is yanked from the husband's butt, and again they have sex. This keeps up for about half the night, until after about the fifth or sixth time, when the wife goes back to her side of the bed.
The husband then rolls over and whispers to his friend, "It's bad enough that your screwing my wife, but must you use my butt for a scoreboard!"
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