Weird you say that because many libertarians have said that have no problem with any kind of marriage even with animals and it was communists which pushed homosexuality to be normal .
Should a person have the right to declare himself to be the life partner of his dog, his Yugo, and the number 57? Sure, why not? Such declaration wouldn't mean anything, but if saying it makes the guy feel better, good for him. However, on the flip side, the fact that the person has the right to declare himself the life partner of those things does not give the person any right to demand that anyone attach any significance whatsoever to such declaration.
It's worth noting that the government's primary interest in marriage stems from establishing the paternity and legitimacy of children resulting from the sexual union implied thereby. Civil unions (including among heterosexual people who would be ineligible to get married) could serve all other purposes which marriage would serve.