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.