Mine is: a masochist and a sadist that get married. Union made in heaven, isn't it?
So, well, they get married and then go for a honeymoon in some expensive Las Vegas S&M dungeon, just for the two of them. When they get there the masochist, all excited, jumps on the bed and asks, desperate "hit me! hit me!", to which the sadist, all cool, replies "no".
Ta-da!