Jacob Levy throws a slap or two in the direction of Sam Brownback’s argument that gay marriage will lead to a declining birthrate. It’s worth noting that, among rich countries, there’s actually a rough correlation between social conservatism and relatively low birthrates. Traditionalist societies like Italy and Spain do relatively little in the way of social policy to ease the burdens of motherhood, whereas highly postmodern countries like Sweden have lots of free day care and other such things. The result is that Catholic Southern Europe has lower birthrates than the secular north. The marriage laws, as Jacob says, have nothing to do with it.
So if boosting birthrates is really what you’re interested in, socialism is the only way to go. Banning gay marriage won’t change anything.