I think part of it is that until the recent explosion of letters following GLBT, bisexuals didn't have their own identity separate from gays/lesbians in pop-culture. That was in part due to people just not understanding, in part due to the tactics used by gays/lesbians to gain legitimacy in the eyes of heterosexuals, and partially because EVERY bi celebrity has had an extremely independent personality (Bowie,Brando,Mercury,etc).

So what I think is happening now, is that bi identity is being formed through internet culture, which largely belongs to people under 20 years old, and since girls have an easier time being bi than boys, it's predominantly teen girls who make the memes and posts.

I do agree with your sentiment OP. This sub skews young and female. It doesn't speak to me as a bi guy in his 20's, and I do feel that my world view isn't represented here very often.