My theory is that people with longer hair tend to face away from the showeer while washing their hair, because they can tilt their head back into the spray while rinsing it. If you face the shower you have to lean forward, and the wet hair and shampooey water run all over your face. People with short hair face the shower.

Statistically, this means that women face outwards, but I'm sure it isn't a protective stance just in case Tony Perkins appears...