The primary argument put forward by those who claim to oppose gay civil marriage for the sake of children is that allowing gay people to enjoy the legal benefits of marriage would deal a death-blow to the already crumbling institution of marriage in American society. Destroy marriage, they say, and you undermine the stability of families, and thus threaten the welfare of children.
My father came out to me when I was eight years old, and I think even at that tender age I would have questioned how anyone could claim to be promoting "family values" by denying some families the basic legal rights provided by civil marriage, rights that other families take for granted. From an early age I found it impossible to make sense of discrimination on the basis of sexual orientation, and I was haunted by the knowledge that my father was so inexplicably denied the full protection of the law. It is the denial of basic civil rights to gay people, not the acceptance of gay civil marriage, which truly threatens the stability of families and the welfare of children. Prominent conservatives like David Brooks and William Safire are among those who have recognized that they must support civil marriage for gay couples as part and parcel of their "family values" agenda. I don't agree with Brooks or Safire on much, politically speaking, but I admire their consistency on this point.
From where I stand, it's the anti-civil-marriage folks who give the institution of marriage a bad name, forcing many members of my generation to question whether we want anything to do with it. Though as a straight woman I have the right to marry, I am not sure that I could stomach taking advantage of that right while it is still denied to my father and his partner and to so many others. Opponents of gay civil marriage are turning the institution they claim to love into the "whites-only" water fountain of my generation, and there is little less appealing than that.
When you realize that denying gay people the right to civil marriage threatens the welfare of children and tarnishes the very name of marriage, it starts to look like a pretty strange way of defending America's families. But then, maybe defending America's families isn't what the opponents of gay civil marriage are trying to do at all.