Biphobia or monosexism is aversion toward bisexual people or bisexuality as a sexual orientation. Biphobic prejudice commonly presents as denial that bisexuality is a genuine sexual orientation, negative stereotypes about people who are bisexual (such as the beliefs that they are promiscuous or dishonest), or bisexual erasure.

The hatred of bisexual women is referred to as bimisogyny or less commonly bisexism.

Biphobia is a portmanteau patterned on the term homophobia. It derives from the Latin prefix bi-(meaning "two, double") and the root -phobia (from the Greek: φόβος, phóbos, "fear") found in homophobia. Along with transphobia and homophobia, it is one of a family of terms used to describe intolerance and discrimination against LGBTQ people. The adjectival form biphobic describes things or qualities related to biphobia, and the less-common noun biphobe is a label for people thought to harbor biphobia.

The term biphobia was first introduced in 1992 by researcher Kathleen Bennett to mean "prejudice against bisexuality" and "the denigration of bisexuality as a life-choice." It has subsequently been defined as "any portrayal or discourse denigrating or criticizing men or women on the sole ground of their belonging to this [bisexual] socio-sexual identity, or refusing them the right to claim it."

Biphobia can lead people to deny that bisexuality is real, asserting that people who identify as bisexual are not genuinely bisexual, or that the phenomenon is far less common than they claim. One form of this denial is based on the heterosexist view that heterosexuality is the only true or natural sexual orientation. Thus anything that deviates from that is instead either a psychological pathology or an example of anti-social behavior.

Another form of denial stems from binary views of sexuality: that people are assumed monosexual, i.e. homosexual (gay/lesbian) or heterosexual (straight). Throughout the 1980s, modern research on sexuality was dominated by the idea that heterosexuality and homosexuality were the only legitimate orientations, dismissing bisexuality as "secondary homosexuality". In that model, bisexuals are presumed to be either closeted lesbian/gay people wishing to appear heterosexual, or individuals (of "either" orientation) experimenting with sexuality outside of their "normal" interest. Maxims such as "people are either gay, straight, or lying" embody this dichotomous view of sexual orientation.

Some people accept the theoretical existence of bisexuality but define it narrowly, as being only the equal sexual attraction towards both men and women. Thus the many bisexual individuals with unequal attractions are instead categorized as either homosexual or heterosexual. Others acknowledge the existence of bisexuality in women, but deny that men can be bisexual.

Some denial asserts that bisexual behavior or identity is merely a social trend – as exemplified by "bisexual chic" or gender bending – and not an intrinsic personality trait. Same-gender sexual activity is dismissed as merely a substitute for sex with members of the opposite sex, or as a more accessible source of sexual gratification. Situational homosexuality in sex-segregated environments is presented as an example of this behavior.