Married or young ladies cannot leave a ball-room or any other party alone. The former should be accompanied by one or two other married ladies, and the latter by their mother, or by a lady to represent her.
--From The Gentleman and Lady's Book of Politeness and Propriety of Deportment--
This is why women always go to the bathroom in groups! It's an old rule of propriety which has managed to survive when so many others have fallen by the wayside.