Bisexual can be used to describe people who have erotic, affectionate, romantic feelings for, fantasies of, and/or experiences with both men and women, and people who self-identify as bisexuals for these or any other reason.
Aren't bisexuals just going through a phase of being confused about their sexuality?
Bisxeuals are people who are attracted to both sexes; their reasons why they are attracted to one sex may be very different from their reasons why they are attracted to the other, and they may not be equally attracted to both sexes.
Howecer, many of us are absolutely certain that we are attracted to both sexes; there is no confusion. It is natural for people who are coming to terms with a sexuality which is not society's norm to feel confused. For some people, bisexuality is a phase between homosexuality and heterosexuality (and the individual in question could be going in either direction); for others it can just be a bried experimentation. But for many people bisexuality is a lifelong, committed sexual orientation.
Are bisexuals equally attracted to both sexes?
Many bisexuals feel they have a "preference" for one gender over another, but they do not deny their attraction for that other gender. Some bisexuals, however, have no such preference, and instead focus their attrations on qualities they see in an individual, regardless of that person's gender. Sometimes these qualities involve gender, sometimes not.
Why would lesbians/gay men dicriminate against bisexuals?
Sometimes bisexuals are perceived as "hiding," a sense that some bisexuals use their bisexuality to look heterosexual at work, in straight acting social settings, to enjoy the "heterosexual privilege" that is part of the social norm. Also, bisexuals are sometimes seen as blurring the issues and weakening the lesbians and gay movement. Naturally, bisexual activists disagree with this view. A further reason is that some lesbians and gay men also have sex with members of the opposite sex (while not identifying as bisexual).