The Volokh conspiracy discusses the ramifications of same sex marriage and it opening the door for the legalisation of polygamy. The dangers of opening the pandoras box of same sex marriage is, I believe, only the beginning of the complete breakdown of marriage.
Canada is already experiencing this problem (same sex marriage has been legalised) with a lawsuit on the way for polygamy to be legalised.
The slippery slope is getting very slippery indeed. While I enjoy Ann Althouse and Volokh, I believe they have missed the boat. The slippery slope will lead to polygamy being legalised. For more see Charles Krauthammer's excellent article.