Use that cubicle

I was at a donut stop recently and needed to use the washroom. So did ten other women. The washrooms were single cubicles - one for women, one for men. The women's had the line up. The men's was vacant. I went to the head of the line and waved the women to use the men's washroom as well. To my surprise, they all shook their heads and said no. Both washrooms were identical - single user cubicles with a lockable door. The only difference was the sign on the door. So I got my relieve in the men's washroom.

Now, what's with that? Why do women line up instead of use the men's washroom when they don't have to share that washroom with men at the same time?