The idea that matrimony leads to children is cultural, not legal. Traditionalists would benefit if they recognized the distinction.
Musing on a new parenting study, Ross Douthat says of same-sex marriage that "the near-universal liberal optimism on the subject notwithstanding, we don't really know how straight culture will be influenced on the long run by the final, formal severing of marriage from procreation."
But why does he presume we're witnessing any such thing?
Straight marriage has long been legally severed from procreation -- that is to say, neither the ability nor the intention to have children is required, and plenty of people who marry never have kids. And even if gay marriage is legal in all 50 states, it'll be entered into by a comparatively small percentage of citizens. Meanwhile, lots of straight people will continue to marry, many of them in part to start families. It's true that we don't really know, in the long run, how straight marriage will evolve (we never have), or if gay marriage will have an effect on it; but as Americans marry and reproduce, as long as pregnancies spur marriage proposals, as long as people "wait for marriage" to have kids, the link between marriage and procreation will persist.