Building for decades, the United States finds itself on the cusp of significant social change, as nearly every day brings fresh news of laws that used to prohibit same-sex crumble away with the demise of the Defense of Marriage Act. One of the primary objections to the legal recognition of same-sex marriage is that it will lead inexorably to polygamy. The spreading legal recognition of same-sex marriages highlights the verge of the slippery slope in stark relief.
Non-monogamy has also been getting a lot of airtime lately, both positive and negative. This is in part because the numbers of non-monogamists are surprisingly high, and appear to be increasing with the Internet making it much easier to find partners and discover new relationship alternatives. Non-monogamy also makes good TV because it is sexy and interesting, and swingers, cheaters, polygamists, and polyamorists have all had their time in the spotlight. It is also terrifying to some people who see it as not only a tremendous threat to their own relationship, but a weakening of the moral fiber intricately wound up with monogamy and sexual exclusivity (at least for women).
All of this attention is for good reason - monogamy and non-monogamy are defining issues of this era. Same-sex relationships are so 20th century, and we have only been there, done that because of the blood, sweat, and tears of hundreds of thousands of activists who fought for gay rights and to make same-sex marriage a growing reality. Many young people today already see monogamy as something to be discussed and negotiated, with a “define the relationship” talk necessary before anyone assumes they are in a sexually exclusive relationship. Even then, they very well might break up and later establish another monogamous relationship with someone else – serial monogamy. Some people negotiate “monogamish” relationships that allow a little sex on the side occasionally.
So what does this mean for society? Will same-sex marriage really pave the way for people to marry multiple other adults, and then on to animals and appliances? Multiple other humans, possibly, and it would not be particularly strange when we broaden the view to cultures outside the US and in the past. In fact, polygamy is the predominant relationship style in many other parts of the world, as well as numerous historical epochs -- nearly always as polygyny with, one man married to multiple women. Polyandry, with one woman married to multiple men, is rare.
But horses and toasters? Not so much. It is a tremendous leap from relationships among adults of the same species that are at least potentially consensual to definitionally non-consensual relationships with mute beings or objects. Other cultures and eras have recognized same-sex couplings and multiple-partner relationships without also wedding their members to animals and inanimate objects.
Far more likely, the vast majority of adult humans will continue to have relationships with other adult humans, just as they are doing now. People who already have sex with children or animals (or objects, without the same ethical issues of lack of consent) will also continue to exist, regardless of the legality of same-sex marriage.
Even if same-sex and multiple-partner marriage are both recognized legally at some future point, they are unlikely to eradicate conventional marriage. Because the significant majority of people are heterosexual, they are not particularly prone to marry someone of the same-sex. Multiple-partner relationships do not appeal to everyone -- monogamy is not only often simpler and less threatening, but it is also the cultural and religious norm. It is most likely that only a select portion of society will actually take the ride down the slippery slope to multiple-partner relationships, and even if they do, it probably won’t change the relationships of people who want to be in monogamous marriages.