William Eskridge and Darren Spedale, coauthors of the book "Gay Marriage: For Better or For Worse? What We've Learned From the Evidence" (Oxford University Press, 2006), have looked at marriage rates and other evidence of the social effects of recognizing same-sex relationships in the 17 years since Scandinavian countries began doing so.

In an op-ed in the Wall Street Journal, (subscriber only) they summarize their findings as follows: Seventeen years after recognizing same-sex relationships in Scandinavia there are higher marriage rates for heterosexuals, lower divorce rates, lower rates for out-of-wedlock births, lower STD rates, more stable and durable gay relationships, more monogamy among gay couples, and so far no slippery slope to polygamy, incestuous marriages, or "man-on-dog" unions. From the op-ed:

Eskridge and Spedale also find benefits to gay relationships:

Fears about slippery slopes, commonly expressed whenever there has been a change in marriage policy, have also proven unfounded so far:

Finally, what about the "slippery slope" argument — that same-sex marriage would start a dangerous movement toward legal recognition of socially unacceptable relationships? This hasn't happened in Scandinavia; 17 years later, there are still no calls for recognizing polygamy, incestual marriage or marriage to animals. Danes you ask about the slippery slope think you are joking. They realize that same-sex marriages serve essentially the same goal as opposite-sex marriages: lifetime commitment to your better half, the person who completes you.