I'm confused by the quote... To oppose same-sex marriage you have to believe that unmarried cohabitation is wrong? That would seem to me to support it more than oppose it.
Personally, I don't believe homosexuality is right at all. I do have an obligation, however, to respect the United States Constitution. Unfortunately for my moral beliefs, the Constitution has no provisions that explicitly disallow homosexual marriages, and my interpretation of that is that same-sex marriages are already legal. While I don't support homosexuality, I do support the Constitution. The only way my mind will be changed on the legality of the issue is if an amendment is added to the Constitution explicitly denying same-sex marriages.
Notice I support the Constitutional legality of same-sex marriages. I don't support the morality, and I definitely would vote in favor of an amendment barring them. However, our government is run by laws, not morality, so the only actions I can take are to disagree and vote likewise. The states have the power to make their own laws within the bounds of the Constitution, and so far same-sex marriage laws have been put in place.
*I added the part below in an edit, and Geru has already posted while I was typing, so this is to avoid any confusion if there is any*
Oh, and Geru, since you got so emotional, I have prepared the following example of the dangers of politics.
Check out the nice men of NAMBLA, the North American Man-Boy Love Association. If I'm not mistaken, your opinion is in favor of sexual freedom for all. Part of the case for homosexual unions relies on the argument that the Constitution does not disallow them and therefore any law barring them is unconstitutional. Child molestation is not included in the Constitution either, so I believe that argument indirectly supports the idea that a 45-year-old man can have sex with his 12-year-old nephew, as long as they both consent. So, while I support the Constitutial legality of same-sex marriages, I raise the point that by taking that stance, one supports many illicit practices. Such things are the very reason why states were given the right to make laws that can disallow certain actions.
Where does sexual freedom for all end? Where do unacceptable practices begin? Johnny got to marry Joe, what's diferent between uncle Bob and me? Do we really want to face these issues, or should we maintain our stuffy, yet effective, system of morals? Last I checked, democracy (yes I know we are technically a republic) has worked for close to 300 years without allowing same-sex marriages. I'm sure you'll argue "Oh, and democracy worked fine with slavery in place.", but look how long slavery lasted compared to the system of morals the US has tried to uphold. Slavery was physical abuse of human beings. Disallowing same-sex marriages is adhering to a system of morals that the public generally agrees on.
Should same-sex marriages be condoned by governments, will churches be mandated to allow them? That strips freedom of religion, for the government would be controlling the church. We may as well make a Church of the United States and make at least bimonthly attendance necessary. Let's all sing hymn 911, "I WILL NEVAR FORGET BUT RENAME LORD OF THE RINGS!@!11"