Our cat's litter box is already in the bathroom. They just come in and leg wrap and want to be petted, regardless of which bathroom I am in. One of my best friend's cats would only climb in her lap when she was sitting on the toilet. Several other friends report this same behavior and no one really seems to know the answer to it. It's not a problem, well sometimes it is surprising (My father-in-law was a little taken aback when he was visited by the cat who had attached to him while he was in the bathroom one morning). It's just an oddity.