There are those who think allowing Gay Marriage harms the "traditional" kind. I've never quite understood the argument that one has much to do with the other and generally, when those who oppose same-sex weddings are asked to explain this, they double-talk and dodge the question.
In any case, statistician Nate Silver has assembled some interesting data that suggests the opposite of this view may be valid. States that are friendly to the idea of two guys or two gals getting hitched seem to have lower divorce rates. Maybe we're in for a raft of commercials that say, "Save Traditional Marriage. Legalize the Gay kind!"