I'm not an American, but the original question was "Christian response to gay marriage" What has that got to do with your country or constitution? As a Christian you may live in America, but you are now part of a greater eternal Kingdom and it is more important that your views as a citizen of the Kingdom of Heaven should match that of your King, not the earthly country you live in.
My view is this, God plainly states what marriage is and what it is not. He didn't ask for man's opinion despite the fact they think they know better. They can call it a 'mariage' all they like. Legally in the eyes of man's law they can say they are married, but in the eyes of God they are not and never will be. satan seeks to destroy the Christian faith in the world and as he has done with many things, gay marriage is just a pale counterfeit of what a real marriage is and what it stands for.
Would I go to a gay wedding? No. But if I have to interact with gay people who claim to be married and the law agrees with them, then I shall treat them respect, love them and pray for them to come to know God and to see the truth. Condemnation will only harden their hearts toward God and it is not my job to judge them or condemn them because they do not ascribe to my beliefs, so they cannot be measured by that standard.