Banning gay marriage via a Constitutional amendment seems about as silly a notion as the government could possibly have. Can you image the wasted time and effort and money? Do we really need the President, federal and state congressmen, and the courts to spend our tax dollars fretting over this matter?
What a shame that a Constitution that guarantees liberties would be used to restrict them. Recall that Prohibition was a restricting type of amendment and look at how well that worked out.
In the end, marriage has always had a civil component. Many people don't even get married in churches anymore, and many more who do aren't actually religious people. Those who do and are religious, well, they have a fine marriage and we wish them the best.
But banning gays from marrying clearly serves no purpose. Some churches would even recognize their marriage as a holy covenant. Marriage is significant for those who have a taste for the sacred or a taste for the commitment it implies. But many others get married while drunk, by Elvis impersonators or while skydiving naked. Many get married routinely. Some get married and have no intention of having children, including the impotent, those who have been neutered or will take birth control, and those who are too old. Marriage isn't sacred or only about raising a family.
My marriage is not threatened by allowing gays to marry any more than my marriage is threatened by a black man marrying a white woman. It used to be illegal for different races to marry. It's time to drop the bias and allow consenting adults to marry and achieve the legal benefits that marriage provides.
I'd probably say the same is true for polygamy. After all, if adults freely enter into the contract of marriage, I see no reason why the state should decide it's okay or not. Again, it's a matter of tastes, whether with one or more partners, interracial or cross gender. And besides taste, homosexuality may have a genetic basis too.