Unless you are willing to prohibit gays from having children, this is an argument FOR gay marriage. Some gays are going to bring in children from a previous straight relationship, others will adopt, still others will use surrogates or in vitro to have their own. Unless you are willing to prohibit all these things - including literally ripping children from the arms of gay parents - or can turn gays straight, this will continue. With gay marriage prohibited, all these children are inherently born to and/or raised by single parents, for we deny them the security and prosperity of a married couple home. If you believe that marriage strengthens a couple's bond, lessens the chance of a split, provides additional security to children, and thereby lessens a child's chance of growing up in poverty - the highest predictor of a failed life in many forms - how can you support withholding this from children of gays?
When I was young I thought gays were completely and bizarrely different from us normal folk, aberrations (if not abominations!) not to be trusted, and was very much against allowing gays to marry, adopt, teach, etc. I never knew one openly gay person; it just wasn't done in rural Tennessee in the 70s, and frankly we doubted that such behavior was more than a handful of people (mostly in California.) Part of this was getting caught up in the S/M scene in Atlanta at a young age and seeing some bisexual people who were, um, pretty messed up, and extrapolating how messed up would be someone who was actually gay rather than bi. It took three friends, a co-worker who was patently a great guy and none of those things but who was gay and a great lesbian couple, to disabuse me of this notion. (If I have known only three gay people and they are all great folks, obviously either my position on gays is defective or my knowledge of statistics is faulty.) Yet even then, as a redneck from the deep woods with all the ignorance and callow certainty of youth, the children of gay marriage bothered me. How could I be pro-family and yet anti-gay marriage? My solution at the time was to simply not think about it and to assume these children were a vanishingly small sample probably best handled by the government. After all, I didn't know any children with gay parents; how many could there possibly be?
Today we don't have that luxury. With surrogates and in vitro and widespread divorce there are many, many children with gay parents. We have to take a side. Are we pro-family and pro-marriage? Or are we anti-gay? The cost of being anti-gay is to deny all these children the security of a married home when absent government discrimination, some would have that security. That's a high price to pay for maintaining the illusion of traditional America, and it's the worst kind of price - the kind we can make someone else pay.