Heterosexism is the belief that heterosexuality is superior to all other sexual orientations (e.g. homosexuality, asexuality, and bisexuality). I include it to mean the belief that heterosexuality is the archetypal sexual orientation, i.e., that originally or deep down everyone is heterosexual but their heterosexuality has been repressed or combined with other feelings.
Heterosexism, in my usage, is not the same as homophobia, believe they are the same. I believe it is possible to view homosexuality as inferior without being irrationally afraid of it. Even if it's not possible, many people believe it is possible so we should have different words for these different ideas.