My Adventure to the Boy's Bathroom

Why do we follow so many culture norms?
If I haven't mentioned yet, I am a school counselor. I absolutely love my job. I have a longer contract than the teachers, so I work during the summer. Today there we're doing reinvations in our main office and had much of it blocked off, not my office of course so I got to come in and work. The main office also houses our private "teacher only" bathrooms, these were blocked off as well. After lunch and 3 diet pepsies later, I had to pee. (Speaking of, what's the pural of pepsi?) This made my adventure to the dun dun dunnnnnn student's bathroom! Yes I know this has been cleaned by the janitors since the students left and most of the germs should be cleaned, but the thought of using something that our unbathed, germy teenagers use makes my stomach cringe. But, when you gotta go, you gotta go.
My walk down the hall was a long one and much to my surprise when I reached the students' bathrooms the girls was closed, locked even! This left my only option to go to the boys. There were no more than 4 people in the whole building so being caught shouldn't have been such a concern, but it was. This is not how we are raised. I felt almost naughty. Could I be arrested for using a boys bathroom when I am a girl? The boy's bathroom looked much like the girls, but somehow I felt like it was dirtier. When I left and was returning to my office I wondered... why do we follow such culture norms? Do they have bathrooms for girls and boys in other countries? Why do women wait in line while the boys stroll through. Why can't we share?
ps if can be arrested for this, let me know.