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.
Yes, you can find advocates for polygamy and other changes in marriage in Scandinavia and among queer theorists and academics in the United States. But there have always been such advocates, going back to the days of the "free love" movement among radicals in the U.S. in the early 20th century. You can find advocates for anything, complete with a Yahoo group and an organization of the like-minded. Google has been a great resource for slippery-slope fearmongering. But the fact is, neither polygamy nor these other destinations down the slope have caught on as serious legal reform as a result of protecting gay families anywhere in the world.
What we can say with confidence so far, based on the evidence, is that the sky doesn't immediately fall when a society recognizes gay relationships. As time passes without the sort of cataclysmic consequences predicted by opponents of gay marriage, we will be able to say more. We may soon be able to say, with good evidence to back it up, that recognzing gay marriage leads to greater stability in gay families, with benefits to gay couples, children raised in gay families, and communities. The signs so far are pointing in the right direction.