Being gay is not the same as being transgender. While transgender people will often use a different bathroom than their outward sex, gay people use the restroom of their gender just like everyone else. Some people don't like public restrooms and will go to great lengths to avoid them in general, too.
I promise you that gay guys are using the bathroom at your school. I went to a tiny rural school and there was only ~3 gay guys in our graduating class, but a few years later, there are people who have come out that were not obviously gay in highschool and that number is 5x what it was.