Logic continues to escape the reasoning of gay marriage opponents. The alternative for gay couples denied marriage is not to split up, marry the opposite sex and produce copious amounts of babies; it is instead to carry on with the status quo. There is no societal benefit in that status quo.
Homosexuality has shown up throughout recorded history and across all cultures in predictable regularity; it is hardly a simple choice that one leaps upon in middle school (at a time when peer pressure can cause teens to follow one another like proverbial lemmings).
The hurdles gay individuals must overcome just to feel comfortable in their own skin is only compounded when we deny them the chance to marry as their hearts and minds have told them to, just as heterosexuals do.
As for the preference of raising children within a happy, heterosexual union: in a perfect world, yes. This world is far from perfect, and many children go wanting for the love and care we deny with laws against gay marriage.
There is no perfect family, but we can keep trying. No ban on gay marriage will produce more hetero-coupled offspring than without, but it does deny society the stability that comes with an increase in the number of married couples.