You write: "Often, the exclusively heterosexual nature of "opposite-sex marriage" will not be explicitly mentioned,"
While I agree that opposite-sex marriage is often simply referred to as "marriage" it should be noted that opposite-sex marriage is not exclusively heterosexual. Many bi people and more than a few gay and lesbian people have participated and continue to participate in these marriages.
You might change to say something like, "due to heteronormative presumptions..."