heterosexism

Lisa: Friends? These are my only friends. Grown-up nerds like Gore Vidal and even he's kissed more boys than I ever will.
Marge: Girls, Lisa. Boys kiss girls.

-- The Simpsons, episode 3F22

Heterosexism, broadly, is discrimination on the basis of sexual orientation, just as racism is discrimination on the basis of race. Beyond that, people use the term in one of the following related senses:

• Language or behavior that denies the existence of nonheterosexual behavior or identity.
• Language or behavior that simply assumes that a given person is heterosexual.
• Discrimination against nonheterosexuals as it is institutionalized in law and society. In this usage, heterosexism is often contrasted to homophobia, the visceral fear or hatred of homosexuals, which occurs in individuals.

• You communicate with people who are lesbian, gay, bisexual, heterosexual, transgender, transsexual, people who feel that they don't fit any of those, and people who don't want to define themselves in any such category. Try not to forget this.
• Remember bisexuality. Don't assume MOTAS applies to everyone.
• When you refer to people in general (rather than specific individuals or couples), say "partner" or "significant other" rather than "husband", "wife", or "spouse".
• When talking about marital status, remember that in most surveys/studies the "single" category generally includes people who are in same-sex couples. When talking about couples, be clear whether you're referring only to heterosexual relationships or not.
• If you hear something that seems unintentionally heterosexist, point it out to the speaker/writer. In many cases, this is a simple oversight, and the person may be glad to be corrected.
• If someone tells you that you've said or done something heterosexist, don't feel offended or accused of being a bad person. Heterosexism is so pervasive that everyone is guilty of it once in a while, straight or not.

Tips adapted from "isms at a University Community, a Workshop" held at Slippery Rock University, 1993.