MYTH: Bisexuals are promiscuous. (“Bisexuals cannot be monogamous” is a variation on this myth.)
REALITY: Just like people of any sexual orientation, bisexual people have a range of sexual behaviours. Some have multiple partners, some have one partner, some go through partnerless periods. This has to do with the individual, and not their sexual orientation. Bisexual people are no more predisposed to be sexually promiscuous than are straight people or gay people. When it comes to monogamy, some bisexual people choose to be monogomous, and some do not. Bisexuality is a sexual orientation, which is independent of the lifestyle choice of monogamy versus non-monogamy.

MYTH: Bisexuals are fence-sitters. They just can't decide whether to be straight or gay. (Variations on this myth include “Bisexuals are just trying to hide their homosexuality” and “Bisexuals are ‘in transition' between straight and gay.”)
REALITY: Bisexuality is a sexual orientation in and of itself, just as heterosexuality and homosexuality are sexual orientations. Asking why a bisexual can't just decide to be homosexual or heterosexual is analogous to asking a person whose mother is African-American and whose father is South Asian why they can't just choose one culture as their identity. Having said this, sometimes bisexuality is a transitional identity for people on their way to adopting a gay or lesbian identity, or a heterosexual identity. Increasingly, people who formerly identified as gay or lesbian are “coming out” as bisexual; in such cases homosexuality is the transitional phase.

MYTH: Bisexual means having concurrent lovers of both genders.
REALITY: Bisexuality refers to the potential for being sexually and/or romantically involved with people of either gender. This does not mean that bisexual people must have lovers of both genders simultaneously, although some bisexuals do.

More on bisexuality….
Bisexuality means sexual or romantic attraction or behaviour directed to members of more than one sex. However, people who identify as bisexual are very diverse in their attractions and actions, ranging from monogamously married, to living in a long-term homosexual relationship, to having multiple relationships with people of both genders. People who are bisexual will often have a preference for one gender over the other in their sexual or emotional attractions. Some bisexuals view gender as incidental, and are attracted to people by who they are, regardless of gender.

By its very nature, bisexuality is a subversive identity, which contradicts the established social constructions of homosexuality and heterosexuality, the categories of “us” and “them”. Instead, it presents sexuality as fluid and changing. It challenges the homophobia and heterosexism of the straight world, and the exclusionary politics of the gay world.