Ok first off, I've seen this as dog in the bathtub on this site and it is wrong. Dogs love water, cats hate water. Which is why when you are fucking a girl in the ass and manage to get your nuts up in her too it is called cat in the bathtub. Have you ever tried to keep a cat in a bathtub? It is about as tough as getting both nuts up a girls ass.

A sexual act involving trying to push your testicles into someone's anal cavity during non-participatory guy on girl intercourse.
Usually done to try to get more enthusiastic involvement from your partner.