I recall having a history professor who presented a very convincing argument that the rise and fall of great civilizations could be linked to the destruction of the family (e.g. mom, dad & kids), and that we are trodding a dangerous path ourselves given the lack of committment to marriage and children demonstrated by adults today. (seriously loose paraphrase, as I'm working from memory here) I don't know how you would link that up with a gay agenda, though.
I do think that society as a whole, straight and gay, has lost sight of the value of stable relationships. It's all about the great "I".
(having my own little off-topic ramble)
I wouldn't rule it out, but it's hard to be convinced by the mere assertion that there's a convincing argument out there somewhere.
I definitely agree in general that there's a disturbing trend in our society of people neglecting, abandoning or devaluing their family ties, including, but not limited to, those with children. Does it not make sense, then, that we as a society should encourage people of ALL genders, sexualities, religions etc. to settle down and cling to and care for their families, whatever those families may look like, and look after the children who need them (whether from their own loins or from someone else's who failed to do so)? Is a fierce political struggle over the right to have our relationships, our families, validated, protected, and supported by the government, the same as those of heterosexuals, not a pretty good demonstration of an understanding of the value of stable relationships? Marriage of course isn't essential to holding a family together, but it sure makes things easier in a thousand and one ways.
In other words, isn't same-sex marriage an intensely PRO-family concept? It is in that it would encourage the formation of stable, lasting families -- they may not look like your family or Beaver Cleaver's, but maybe one reason some people stop appreciating and nurturing their family ties is this constant conservative war cry that "THIS is what a family is SUPPOSED to look like -- anything different from THIS traditional ideal of family is a disgusting, illegitimate mockery" -- such that if someone happens to be attached to a family that DOESN'T look like that (a young pregnant unmarried couple, a single mother, queer folks) it's easy to say, "Huh, guess this isn't really a family so it doesn't matter to me." It's just a conjecture, but it would hardly be the first time conservatives were actually causing the problems they profess to combat. I've yet to see any of the crowd shrieking that gay marriage will destroy (heteronormative) families give a clear explanation of how exactly this would happen -- are all the mommies and daddies going to wake up one day and say, "Wait a minute, there are people of the SAME SEX out there somewhere MARRYING each other? Screw taking care of our kids! Let's flee to Bora Bora until this blows over!"?