Bisexual women and men cannot be defined by their partner or potential partner, so are rendered invisible within the either/or heterosexist framework. This invisibility (biphobia) is one of the most challenging aspects of a bisexual identity. Living in a society that is based and thrives on opposition, on the reassurances and "balanced" polarities of dichotomy affects how we see the world, and how we negotiate our own, and other peoples lives to fit "reality."
Most people are unaware of their homosexual or heterosexual assumptions until a bisexual speaks up/comes out and challenges the assumption. Very often bisexuals are then dismissed, and told they are "confused" and "simply have to make up their mind and choose." For bisexually identified people to maintain their integrity in a homo-hating heterosexist society they must have a strong sense of self, and the courage and conviction to live their lives in defiance of what passes for "normal."
The institution of heterosexism is based on a mutually exclusive heterosexual/homosexual framework. This heterosexist paradigm posits two sexual orientations on either side of a "fence" that draws the line where privilege begins and ends. Heterosexuals are on the "normal/good" side and homosexuals are on the "abnormal/evil" side. The line separates and protects "us" from "them," while it assures members of each side of what they are not. This line also effectively marginalizes lesbians and gay men as "other" and is the core of homophobia.
Furthermore, lesbian, gay, and heterosexual people are invested, and find a sense of security in being the "other" to each other, and unite in the fact that they are only attracted to either the "same" or the "opposite" gender/sex. This sets up another "us" vs. "them" dynamic which effectively marginalizes bisexual people as "other." Integral to this dynamic is the automatic assumption people can be defined by the gender/sex of their current or potential romantic interest. For example: two women are assumed to be lesbians in a "lesbian" relationship; two men are assumed to be gay in a "gay" relationship; and a man and woman are assumed to be heterosexual in a "heterosexual" relationship. However, any, or all of these people could be bisexual. And depending upon monogamy and non-monogamy agreements and choices, any, or all of these folks could have sexual behavior with more than one gender/sex whether they identify as bisexual or not.