I mean, yes, but (in my experience) that’s a niche term used by people already fighting for equal rights, not people who think gay marriage will pull the country apart. The type of person who uses “not wanting gay people to get married doesn’t mean we’re scared of them” as a counter-argument won’t have heard of heterosexism.
Maybe it’s the area I live in (conservative Christian rural middle America) but I see that “we’re not SCARED of them, we LOVE them!” excuse of an “argument” all the time because homophobia is still the first used word, and it’s easier to say that than to explain why they think it’s okay to impose their religious beliefs on other citizens in a country that isn’t supposed to favor any religion.
But yes, you’re right, that term does exist, as relatively unknown as it is.