I remember when I was at school, I wasn't so much looking for a chance to embrace an alternative "lifestyle" like homosexuality as I was desperately trying to convince myself that I was 100% heterosexual. Choosing to engage in homosexual behaviour still requires that a person enjoy same-sex sexual conduct, which I think requires something other than merely a rebellious attitude to mainstream norms.
I actually ran into a guy two weeks ago who said he had a "gay phase" while in high school, but was now married. I would label such people as bisexual, although they probably wouldn't.

There are people who are attracted to both men and women, not always evenly, but we're the exception rather than rule when it comes to same-sex attraction I believe. Our existence tends to get obscured by (a) gay people who view heterosexuality as the enemy and insist people pick a side (b) gay people who used a bisexual identity as a stepping stone to embracing their true sexuality and mistake the bisexual label as always meaning "gay but still partially in the closet", and (c) people who view all people that engage in same-sex sexual activity as "homosexual", regardless of whether the people involved also have some opposite-sex attraction or not.
I am a bisexual man with a strong preference for men. I could "choose to be straight" and go out only with women, or I could "choose to be gay" and go out only with women. I don't pick a side because I don't see the point, and in any case the attraction to both genders would still be there. Most people don't have that luxury of choice.