One the one hand, there's the doctrine of equal rights and non-discrimination. On the other hand, there's the sanctity of marriage, and the slippery slope. Marriage has always been recognized as a holy union between a man and a woman, and allowing "alternative definitions" of marriage to be recognized, it's only a matter of time before group marriage and unions between a czarina and her noble steed attain the same recognition.
Opponents to gay marriage cite that homosexuality is an abomination before the lord. On that ground, it should be outlawed, and the mere idea of gay marriage doesn't even merit debate.
Fine. Then outlaw shrimp first. Public stonings of adulterers will then be regularly scheduled on alternating Tuesdays in front of the Washington Monument.