When an arthritic man and a woman love each other very much they have crab sex. It is called crab sex as the arthritic man will only move awkwardly sideways, not up or down.
Guy1: (pauses game of quietpes
)Whats that noise coming from upstairs?
Guy2: Thats my housemate with his girlfriend
Guy1: That doesn't sound right, he's doing it wrong.
Guy2: No he's got arthritis remember, it's crab sex