That is exactly what the 'homosexuals' are attempting to do.
Now there are only two courses open, since if a state recognizes a union of this sort as a 'marriage' all states must also accept that recognitation.
One, outlaw it on the state level as is polygymy (which won't happen-but should) or two, define marriage on the federal level so that no state can redefine it.
The point is that the federal level must protect all states from one state abusing the union we have among ourselves.
Well, that would protect the word 'marriage,' but in either case it would be denying some people due process. The due process I refer to is following the proper rules for gaining legal access and recognition.
If the attempt fails, as I mentioned it might, at least the group cannot claim deprivation of rights or denial of due process. Let the people decide. Not the courts or Congress. Else we're going to have a group of some very unhappy people out there who are going to feel dis-enfranchised. We have a system that will insure equality to all. Give them an opportunity to use it, without destroying the meaning and institution of marriage.
Maybe in another hundred years, the people might get used to the idea of homosexual 'relationships' and regard them on par to married folks and the Jim and Jim Crow laws will fade into oblivion. ;>
I've suggested in the past they use another word to describe their relationship. Even a fabricated word, such as 'smerge,' for instance. If they want to get smerged, let them get smerged and live in a state of smergiage. If they want to smerge each other and be on legal par with similar partnerships, let them push for an amendment that will recognize and ackowledge their class as smerged folks.