Yeah, that was a mystery to me until somebody pointed out that gay marriage does, in fact, threaten a very specific type of hetero marriage. The type social conservatives are most likely to want to uphold - the uber-patriarchal type in which gender roles determine hierarchy within the family (aka "traditional marriage"). If you start out with two people of the same gender who occupy the same social status, you set the stage for what is an egalitarian union on its face. Egalitarian unions are the threat. And as far as I'm concerned, "traditional marriage" should be destroyed, because it's oppressive and supports and sustains the larger oppressive social systems.