Culturally Speaking: Why do gay people hide behind the term bisexual?

It is my belief that so called bisexual men are really gay men who are afraid of coming out as fully gay. And even if you like both you always like one thing more than another. The same goes for so called bisexual women. Why not just be gay, why hide under bisexual? That's just like newly made Atheists hide under the label Agnostic.