The ammendment does not act to limit the actions of individuals, they can shack up and call it whatever they like.
By defining marriage to one man/one woman it limits the federal government (and judiciary) from extending federal recognition and benefits willy nilly to any couple that demands it.
Had the "capitation tax" in the original Constitution not been mucked with, and had the government not unconstitutionally started taking money from one group to give to another, these things would not be an issue.
I have lots more thoughts on the issue, but I doubt that anyone would care to hear them.