The slippery slope argument.
Perhaps the best (or worst) example is the scene from Animal House in which an attempt by the student disciplinary council to discipline Delta House is turned into, through a series of slips and slopes, an alleged attack on the United States of America. Slippery slope arguments have a way of ending up in absurd places, but the absurd end result does not negate a legitimate starting point.
In the same-sex marriage debate, there were some stupid slippery slope arguments, for example, that eventually you would have a right to marry your dog.
But when it comes to polyamory, the slippery slope argument had a sound footing and went something like this: Once society moves away from the traditional definition of marriage as being between one man and one woman, why stop at one-and-one? If loving each other and having a healthy household in which to carry on one’s life, and in some cases to raise children, is the standard, then on what basis does one deny three or more people who meet those qualifications the right to legal recognition by the state as a marital unit, with all the benefits that accrue?
The polyamory slippery slope argument was met with derision precisely because it raised a legitimate point. There are ample historical and modern examples of societies which recognize polygamy and less frequently, polyandry. A couple of years ago Newsweek ran an article arguing that polyamory was the wave fo the future here, accompanied by a photo history of polyamorous societies. ABC News ran a similar article, in the context of the debate over Prop. 8 in California.
The counter argument, that polyamory is a choice but homosexuality is not, is questionable. Given the high rates of infidelity, particularly among our male politicians, there is every reason to believe that people are hard-wired not to be monogamous. What better for society, the argument would go, than to give that non-monogamous tendency an outlet within the family unit?
The legal argument in favor of same-sex marriage, which breaks a marriage down into pieces and then attempts to demonstrate that there is no rational basis to distinguish same-sex relationships, applies with equal force to polyamorous relationships, if one is going to be intellectually honest about it.
Don’t take my word for it. Listen to this audio clip of a talk by Martha Nussbaum, Professor at the University of Chicago School of Law and one of the most prominent intellects advocating for same-sex marriage. (You will find the full hour and 25 minute audio at the link, but I’ve clipped the ending part where she discussed polygamy and incest.) Nussbaum argues that most if not all of the arguments in favor of same-sex marriage apply to polygamous relationships, and even goes so far as to argue that modern genetic science demonstrates that there is no rational scientific basis for barring marriage as to some incestuous relationships.
Once society decides that one-man, one-woman no longer is the “arbitrary” standard, on what rational basis do we stop at one-and-one?
This is not a slippery slope argument against same-sex marriage, at least not in New York State. Same-sex marriage has been signed into law and will become effective in less than 30 days.
Why stop at one-and-one?