I am categorically against gay marriage, but because our society has become so saturated with sex as purely recreational, and not sacred at all, there is not one leg to stand on from a "what God intended" perspective.
God did not intend for divorce, co-habitation, and multiple hetero-sexual partners, either, but it's done and pretty much celebrated all over the world.
The "slippery slope" that we (and I include myself here) claim will happen if gay marriage is legalized, it did not begin with gay marriage. It begins with every single one of us who does not live up to our vows, to stay together, to forsake all others, etc., whether taken before God, or Mother Earth, or whatever.
Multiple partners would have been deviant behavior generations ago. Gay sex just 30 years ago, and today pedophilia. That will change, but not because of gays, but because of us.
It's pathetic how all of us (once again, I include myself) have so much to say about others' sins, or misbehavior, but can find any and all justifications for our own.