Sociologist Peter Berger writes about the increasing popularity of polyamourous partnerships and marriages--relationships which involve multiple people. Among his examples, we hear of the Unitarian Universalists for Polyamorous Awareness, an organization created in 1999. While he is not one of us, Berger concedes that cultural conservatives appear to be empirically correct that "once you legitimate same-sex marriage, you open the door to any number of other alternatives to marriage as a union of one man and one woman."
The moral theory underlying the push for gay marriage is what we might call "consensuality"--if a union is voluntary, it is good. Following this principle, the list of possible arrangments is limitless. It is an invitation to moral choas, a road we have been on for half a century.
I was stunned the other day when a very conservative friend of mine said she didn't have a problem with Mormon polygamous families. I didn't ask, but I presume she would be cool with Muslim polygamy as well. Moral libertarianism--or perhaps lifestyle liberalism is a better term--cripples conservative thinking.