Much as I'm opposed to gay "marriage," I've never seen this claim adequately defended (and it's usually not defended at all, but simply asserted).Did you happen to read this thread about gay marriage in Holland?:
http://www.freerepublic.com/focus/f-news/1157360/posts
During the mid-1990s, the rate of out-of-wedlock births began to shoot up. By 2003, the rate of increase nearly doubled to 31 percent of all Dutch births. The U.S. rate is already that high, or close to it ... so if same-sex "marriage" destroyed marriage in Holland, marriage here has already been destroyed.