I am readily acknowledging that bisexuals actually often are discriminated against in straight and gay circles, when it comes to mating. The reason is as simple as it can be: nobody wishes to engage in an emotional relationship with a person who might ab initio "miss" something in this relationship. Prejudice or not, the mere thought that my boyfriend might as well easily land in bed with another woman, seems particularly distastefull to me as a gay man.
Beyound this, 2 of 3 self-professing male bisexuals I personally know are older men who were married to women previously, and now feel more attracted to (much younger) men. That's what one might judge as the "fluent" sexuality changing with years, whereby the question is allowed whether the guys were not simply closeted homosexuals before? But again, those folks can certainly much better assess their own experiences and orientation, and it is far from me to put them into one or other drawer to regain order in my simple world view.
What I object is the notion introduced by queer theorists that both gender and sexual orientation are merely a question of social discourse, thus gender and sexuality were "fluent", "volatile" and "interchangable" in principle. While I again readily acknowledge that this might apply for some individuals, I am inclined to persist that this is obviously not true for all or even most individuals. While the projection of what applies for majority to all human beings is intrinsically wrong, it is no less wrong to extrapolate the "truth" of a minority to the entire rest of humanity as well.
While some people imagine an ideal world as being one with as few diversity as possible (to avoid division and mutual hostility), I am among those who appreciate the differences. It is OK for me to live in a world consisting of hetero-, bi- and homosexuals, of males, females and transgenders and morphological/genetical intersexuals, and watever other facettes of identity are there in the wide, complicated world. The simple truth is that whereas there might be some "continuum" in gender and sexual orientation, this imagined "continuum" is purelly a statistical one. The single individual can very well identify him/her/itself as belonging to one or other group without much difficulties; the group interest are often as clearly defined and might differ significantly.
In case of necessity, humans still can form alliances whatever their particular identity might be -- without blurring the differences.