None of this would be a big deal if
1) The US made up a new term for marriage to distinguish it from the xtian rite.. Call it a 'confluence' or something. So a man and a woman would go to church for a marriage, and go to the court for a 'confluence'!
2) The law was very mild in giving taxation and legal favors over and above a regular nonmarried couple. So this should entail mild priviledges to citizenship requests, and visiting rites, but not much more that could potentially weigh on the rights of the taxpaying community.
However, I do still see the slippery slope problem assuming 1/2 above are not implemented. Polygamy/incest/beastiality/whatever could, under the same logic, be fought for as well. With that, comes absurd possibilities for tax loopholes, and undamped ponzi schemes.
So I ask, why should the will of the majority be supressed for something that is not an inalienable human right (marriage).
Gay and Lesbian groups should be fighting for implementation of the above, at least the logic would be clean and not arbitrarily contrived.