Raison de quack quack is inspired by the raison d'etre ... For ducks that would be, to be is to quack. For humans, to be is to ... quack like a duck? Or something else? (1) Being horny and (2) in the mood for a good time.
Queef really liked his women. He definitely had ze raison de quack quack.
"Do you have ze 'raison de quack quack'?", she asked, pulling down his zip.