One night, Robert goes to his mistress's house. "Rachel," he says, "tonight we're gonna have sex, wild and passionate!"
"Sorry honey," Rachel says, "not tonight, I have my period."
So Robert reluctantly goes to sleep.
Several hours later Rachel wakes up to get a drink. On her way back to bed, she notices Robert wide awake staring at the ceiling.
"Honey, I told you, not tonight," she says, "you might as well go to sleep."
"I would," he replies, "except my dick's so hard there's not enough skin left to close my eyes!"