As with much of our theory work, we must define terms first.
David Halperin has thirteen definitions I would like to share: he defines bisexuals as people who ” are sexually attracted to males and females; are not prevented from being sexually attracted to anyone because that person is male or female; are sexually attracted to the individuals they are attracted to, whether those individuals are male or female; are sexually attracted to their own sex but have a sexual history that includes sex with persons of the other sex; are sexually attracted to the other sex but have a sexual history that includes sex with persons of their own sex; are in a stable, long-term, sexual and erotic relationship with some of their own sex but are also sexually attracted to persons of the other sex; are in a stable, long-term sexual and erotic relationship with someone of their own sex but are also sexually attracted to persons of their own sex; have sex only with persons of their own sex who are gay and persons of the other sex who are heterosexual; have sex only with other bisexuals (men or women); have sex only with persons of their own sex but identify as bisexual; have sex only with persons of the other sex but identify as bisexual; have sex with males and females but identify as gay or lesbian; and have sex with males and females but identify as heterosexual.”
As you can see, many people identify as bisexual when forced to choose because of course society can’t just let you be whatever you want or can’t admit that sexuality is fluid. The bisexual identity does serve to undermine the binary of heterosexual or homosexual in a way that it helps others define you. Of course, as a bisexual, you will be defined and treated as heterosexual or homosexual according to your current relationship no matter what your prior sexual history. The binary is not totally destroyed, and bisexuals can feel unwanted and unrecognized by both heterosexuals and homosexuals as well as being portrayed as merely indecisive. I am here to say that yes, bisexuals exist.
Sex, Culture, Politics 
