There is an interesting, and still ongoing, debate over if registered partnerships and gay marriages have killed off heterosexual marriage in Scandinavia.
The anti-marriage-equality case was best presented by Stanley Kurtz (who also opposes marriage-lite measures like civil unions), in a Weekly Standard article.
Both Galios and Andrew Sullivan responded to this article, and then Kurtz responded to Sullivan, and then there were the counter-responses; since I’m lazy, rather than recreating the links, I’ll just point you to this Galios post.
Darren Spedale also wrote a good response to Kurtz.
That was a little while ago; in what might be termed “round two” of the debate, economist M.V. Lee Badgett wrote a response to Kurtz. Then, Galios discussed Badgett’s article, expanding on some points that were relatively buried in Badgett’s piece. Kurtz responded to Badgett yesterday.
I’m sure there’s more on the way.