Actually, my youngest sister went to school with a kid who started to act flamboyantly gay in the fifth grade. He wore lipstick and would constantly comment on how attractive the other boys in class were, etc. I wouldn’t say it’s common, but it does happen, just as some heterosexual kids begin to act overtly sexual at a young age. It’s often (as it was in this boy’s case) related to childhood abuse.
That said, you’re absolutely right that 99% of gay kids aren’t going to act like that, and this girl probably has a terrific case against the school district.
As for the gay male perspective on the locker room situation: I had a gay male friend in high school who informed me about what all the guys in our class looked like naked. He’d checked them all out surreptitiously in the locker room. But he wasn’t standing there ogling them. I think he was curious but definitely not turned on, even by the guys he was interested in, as locker rooms are inherently unsexy environments. And yes, most people knew he was gay.