I live in a country that has legalized gay marriage a few years back. Straight marriages haven't somehow devaluated, no one has come to indoctrinate me, no one has called for polygamous or pedophilic marriages and God hasn't smitten us yet. Why would this be any different in America?
Also, if there is supposedly no difference between marriage and domestic partnerships, why does an artificial distinction need to be made between the two? Either call them both marriage, or neither.