Great Slate article by M.v. Lee Badgett on how conservative claims that gay marriage in Scandinavia has hurt straight marriage
are full of hot air. Lots of great statistics.
Personally, I think the biggest threat to the institution of marriage is the social pressure conservative culture puts on people to enter it. Before gay marriage, the legal ease of getting a divorce was the whipping boy most often made to stand in for failed marriages. A quick spin through such 19th century American classics as The Age of Innocence
easily shows how oppressive divorce laws were. If shruggingly easy J.Lo-style divorces are the price society pays to give women a reasonable exit from abusive marriages, more power to J. Lo. There's nothing more American than the idea that instead of tolerating a desperately wrong situation, one should try to change or escape it. In my humble opinion (and this is all based on observation of my friends, not personal forays into the realm of the engaged), if you want a sacred institution, you have to give couples the space and time to develop a relationship worthy of being called sacred. History has shown that ceremony alone will not do the trick. If a little cohabitation and even a baby is required, that is what is required. But I feel much better going to a wedding where I know the bride and the groom are already married in their hearts than I do going to one where they see the wedding ceremony as the beginning of a serious relationship. When less people get married, we can take the ones who do get married a lot more seriously.