Of course a person at 15 can decide if they're gay or not. I knew by age 5 that I liked girls. Had I liked boys, I woulda been chasing *them* around the playground trying to kiss them. Christ, if by 15 you can't figure out whether you like guys or girls, you may wanna get a CAT scan done, or just finish the process of becoming a eunich and get yourself castrated. There's nothing hard to swallow about a 15 year old declaring herself a lesbian.
But yes, there is a horribly obnoxious new trend of bisexuality that never really gets acted on. I know at least a half dozen 'bisexual' girls who basically once made out with a friend of theirs, and declared themselves bi because they know damn well that when a guy hears that, he goes "THREESOME! AWESOME!" and gravitates towards her. I mean, you're probably right about people hiding it less now, Jeff, but only in the case of guys. Male bisexuals or homosexuals, sure, yeah. There's seemingly more of them because it's more socially acceptable. But it's still not really considered a good thing, like lesbianism is.
It sucks. Bisexual girls used to be such a thrill. Now they're just boring.