[Bisexuality] is seen as threatening the homosexual/heterosexual....

“ [Bisexuality] is seen as threatening the homosexual/heterosexual and male/female dichotomies, or binarisms, which underpin our gender and sexual identities to such a large extent. In the case of the first three stereotypes, there is a refusal even to acknowledge the existence of bisexuality. It is simply wished out of existence. You can either be homosexual or heterosexual but anything else is just a phase, just playacting, not real. As Udis-Kessler argues [‘Challenging the Stereotypes’, in Rose and Stevens (eds), Bisexual Horizons: Politics, Histories, Lives. 1996. London: Lawrence and Wishart, pp. 45-57], this reflects an ideology of essentialism which dismisses the idea that sexuality may be fluid, not fixed, and that its forms can change over a person’s lifetime. This ideology assumes that there is a ‘true’ sexuality which we are working our way towards and that bisexuality is not really ‘true’ or ‘serious’ because it is a transition towards that other state… As Udis-Kessler points out, transitions are not a rehearsal for life. Life is a series of transitions: points of arrival become new points of departure, and vice versa. So why should we assume that the way we experienced our sexuality ten or twenty years ago is necessarily less ‘true’ or important than the way we experience it now, or that the way we experience it now will necessarily be the same in ten or twenty years time? Obviously this applies not only to bisexuality, but it is an argument which those - including some lesbian and gay activists - who accuse bisexuality of being a sort of ‘false consciousness’ seldom get to grips with… lesbians and gay men, anxious to create safe spaces where they are not subject to homophobic rejection or oppression, may (consciously or unconsciously) seek to exclude bisexuals[…].Unfortunately, as soon as this happens, as with every oppressed or stigmatised group, it can lead to others being oppressed or stigmatised in turn. ”

Richard Dunphy

From : Sexual Politics