Women go to the bathroom in groups because of the social aspect. They can talk, share make up, etc. There is also the aspect of not wanting to be alone--whether that's at the table or in the restroom.
The saying "Men are from Mars, Women are from Venus" originates from old Roman mythology. Mars is known as a god of war, and Venus as a goddess of love. Therefore the saying is giving the classic gender profiles of men are rough and dominate, and women are gentle and loving.