Many gay men, when first coming out, went through the "bisexual phase
As I'm sure you know, in this world we live in, it is very difficult to admit to yourself that you are gay. I remember my countless nights of denial (see I'm not gay), and that's what lets me understand this phase.
It's not to say that all bisexuals will "become gay", or that all gay people will go through this phase. Deep down, I always was attracted to men, but I never really wondered if I was attracted to women. I assumed I was, because most men are. So when I first accepted my attraction to men, I called myself bisexual.
However, months later I came to the realization that I -only- thought about men. Somehow, being bisexual was more comfortable than being gay at first, so it was my baby step to coming out. But finally, like many other gay men, I could no longer deny that I just was not interested at all in the female form, and that is what led me to my current sexuality.
However, it should be noted that not all bisexuals will become gay. The idea of a "bisexual phase" is not to say that bisexuality is strictly just a transitional phase. However, in some cases (as in mine), this stereotype is true.