The idea that heterosexuality and heterosexual behavior patterns are accepted as "normal" and that things that are not heterosexual, like homosexuality or bisexuality are aberrant.

If my postmodern anthropologist wife were here, I could tell you who coined the term and when. (She is here--she tells me the term was coined by the poet and author Adrienne Rich.) It's used a lot by queer theorists and other social scientists to describe the bias of our current society and societal mores, and the implicit assumption that people who act a certain way are wrong.