It's not always about the dogs
If I call a tail a leg, how many legs does a dog have?

It's an old question. The answer is four. It doesn't matter if I call a dog's tail a leg. It's still a tail.
This pretty much sums up my attitude toward "gay marriage." Some will call it "marriage" but it's not.
I do think that gay couples should have some legal access to the privileges appertaining to marriage but those are best codified in some form of civil union.
The legal aspects of marriage are what society has evolved to deal with the basically religious institution of "holy matrimony." Society (broadly and vaguely defined) has a real interest in confirming the basic unit of household and procreation as it has been through history. There is the presumption of parentage for example. A poor example for the consideration of gay marriage per se but rather crucial in the world of humans where children fathered by an interloping male have been produced since the memory of man runneth not to the contrary. Society is well served when there is a male in place to whom fatherhood can be ascribed. It makes succession easier (and heritability is one of the bedrocks of functioning society). It may make a lie of a family tree in a genetic sense but society is less concerned with the genes and more concerned with stability.
Even though many male-female marriages are made wherein there will be no children produced (for whatever reason: age, sterility, desire for childlessness), in a very real way the possibility of reproduction exists in any union between a man and a woman. And that possibility does not exist in a homosexual union without the necessary intervention of a third party. Even if the third party is a turkey baster.

And my final thought: Society has evolved a definition of marriage which encompasses the totality of experience of humans since vows were first exchanged. It is a more fluid institution today than it was even decades ago. Divorce is more common and in many ways easier now. Some would say too easy but that's another subject. But what marriage is comes from those who choose it and how the world accepts their choice. There is a very concrete norm for the idea we voice as "marriage." And it is not for those at the fringe to define what is the norm. If gay couples want to define marriage-like civil unions as "life couplings," or "soul unions," or any new term they wish, fine. Marvelous in fact. I'll sign the petition. But it is not their right to change what society (again with the vague and ill-defined! Ed.) recognizes as gathered into the word "marriage."

And thereby hangs a tail.