I will say it again, it is not a matter of what the law allows. If two people want to go to a hyper-liberal church and have the pastor perform a marriage ceremony and pronounce them man and wife, or man and man, or man and wife and wife 2, so be it. Nobody is stopping them from doing this. They're just not going to receive the same benefits from the government for doing so. And equal protection does not mean equal provision.
Take my charitable tax deduction analogy. I am free to donate money to whatever cause I want---well, save a random terrorist organization here and there (a subject for another thread, please :)). But I can only deduct on my tax return those charities that are registered non-profits under appropriate U.S. Treasury codes. I am not being prevented from doing anything: I'm just receving the same encouragement.