Cathy Young has some interesting observations. Some key excerpts:

In 1996, Jonathan Rauch wrote that if same-sex marriage is to succeed, it must become the general norm in the gay community, not just another lifestyle option. At least so far, that does not seem to be happening in Holland.

Also in the past 10 years, the overall marriage rate has dropped, from 5.4 per 1,000 inhabitants in 1994 to 4.5 per 1,000 in 2004. More heterosexual men and women are entering into civil partnerships — which are much more easily dissolved — instead of marriage; in 2004, about 7% of new male-female legal unions were civil partnerships. This does not prove, as Stanley Kurtz has argued, that same-sex marriage undermines heterosexual marriage; the drop in marriage rates is undoubtedly due to many complex factors. However, one can plausibly argue that the changing attitudes toward marriage that make same-sex marriage possible may also be related to overall lower marriage rates. . . . .

Why am I pointing this out? Because, while I fully support legal rights for same-sex partners, I think both sides in the marriage debate have been prone to unwarranted and exaggerated claims about the social impact of same-sex marriage. The legalization of same-sex marriage has not, as some have claimed, led to polygamy in the Netherlands. But at least so far, it has not created a "marriage culture" among gays and has not boosted marriage among heterosexuals. As we continue our own discussion of same-sex marriage, we need to have all the facts on the table.

UPDATE: Dale Carpenter has a thoughtful response in the comments, and Cathy responds in turn.