Heterosexism is the systemic bias towards heterosexual people and relationships with the denigration and stigmatization of nonheterosexual relationships, people, behaviors, and communities. Heterosexism assumes relationships with two partners of the opposite sex as the norm and superior.
Heterosexism is distinct from homophobia because it doesn't always involve overt or active disdain for those who are not heterosexual. Instead, it operates on an assumption of heterosexuality as the norm in all situations and places those who are not heterosexual into the status of "other." Heterosexism can be observed as an element of homophobia.