You are right.
HOWEVER....
I cannot imagine the Founders envisioning a scenario where the "full faith and credit" clause would be used to force this immoral situation down a state's throats. And that is what it is coming to...particularly with the Øbama Administration's backing of the homosexual agenda.
That's why I said, in the OP, the only way out of this mess is to get a Constitutional Amendment in place.
Without that, it is just a matter of time until you, pastor, are going to be required by law to solemnize homosexual "marriages" (You act as an agent of the State when you sign off the marriage license, after all). Or get out of the marriage business altogether. See this editorial if you don't believe that this is the next front in their war on Christianity.
If this were to happen, American Christendom would be well led to unite and establish a common, traditional matrimony. I would gladly join with Catholics in this endeavor. That's not to say that we would conduct one another's marriages or even approve of one another's doctrines of marriage, but we would agree that matrimony is one man and one woman, and that those not receiving an agreed upon joint document would not be married in the eyes of historic Christianity.
I've not thought my way through this at any depth...just initial meanderings on the subject.
A marriage amendment would put the subject in the Constitution at the Federal level, but those are exactly the people I do not trust.