Ok. I'm not trying to show you that homosexual marriage is unconstitutional. For the straight up answer to your question: I think that if you just look to our governing documents, homosexual marriages could fit in just fine.
Now with that said, this is my problem: I dont think that because it may fit into the Constitution that we should allow it. Whether we should allow gay marriages in the United States is not the same as whether outlawing gay marriage is unconstitutional. There is more to the first question than to the second. So I agree that if you just look only to the Constitution, gay marriage fits. But I want to point the other question to you: Should we, as Christian Americans, allow gay marriage to be legal?