This is one I remember from the old Reader's Digest days:
An elderly couple, married 50 years is out on the porch, each in a rocking chair.
Suddenly, the wife stops, looks at her husband, and proceeds to knock him out of the chair onto the porch. Stunned, he gets up and asks "What was that for?" She replies, "That's for 50 years of bad sex"
He gets back in his chair, and after about 10 minutes of rocking, he stops, looks at her and then knocks HER out of her chair onto the porch.
She gets up, dusts herself off, and asks "What was THAT for?"
He replies, "That's for knowing the difference"