Bisexuality only is a transition phase for people coming out as gay or lesbian.
Likewise, some people who now identify as bisexual previously identified as gay or lesbian. Other people identify as bisexual their whole lives. For some people, their experience of sexuality is fluid, something that can change over the course of their lifetimes. Bisexuals are equally attracted to men and women. But many people who identify as bisexual are be more attracted to men or more attracted to women.