There has been the argument that the legalization of same sex marriage would open the way for polygamy. It seems that these fears have been justified. John P. George writes concerning the beyondmarriage.org movement:
For years, critics of the idea of same-sex “marriage” have made the point that accepting the proposition that two persons of the same sex can marry each other entails abandoning any principled basis for understanding marriage as the union of two and only two persons. So far as I am aware, our opponents have made no serious effort to answer or rebut this point. Their strategy has been to dismiss it as a mere slippery-slope argument (although the truth is that it is a more fundamental type of argument than that) and to accuse us of engaging in “scare tactics.” Some have even denounced us as “bigots” for suggesting that same-sex relations are on a par with polygamy and “polyamory”—the union of three or more persons in a sexual partnership.
That was then; this is now.A group of self-identified “lesbian, gay, bisexual, and transgender and allied activists, scholars, educators, writers, artists, lawyers, journalists, and community organizers” has released a statement explicitly endorsing “committed, loving households in which there is more than one conjugal partner.” Got that? More than one conjugal partner. Read More.