Roderick Long reminds us how the institution has already been redefined – and redefined – and redefined:
It is true that the term "marriage" has traditionally been applied, for the most part, to heterosexual unions specifically (though often polygamous ones, a fact such critics persistently pretend to overlook). But it is also true that the term "marriage" has traditionally been applied exclusively to relationships in which the husband held legal authority over the wife – relationships in which the wife was not only subordinated to her husband but actually absorbed into his legal identity.
If we are going to appeal to traditional usage to deny that same-sex partnerships are genuine marriages, then by the same argument we will have to deny that relationships between legal equals can count as marriages. In the traditional meaning of "marriage," then, there are no married couples in the United States today.