The simple reason is that a bathroom used by men is dirtier. It doesn't have to be, but it is! Get your act together, guys! It's a wonder women are willing to live with you at all.
I will give her this much, guys have a tendency to leave the seat a little damp. Outside of that, womens' rooms are much, much dirtier. I speak from experience. From ages 16 to age 25, I was employed in retail industries in some way, shape, or form. I had to do numerous bathroom checks/cleanings, and without fail, the womens' rooms were always dirtier. And disgustingly so at times. I'll spare you all the details...unless you ask for them.