A common argument against the recognition of same-sex marriages is that it will head us down a slippery slope to polygamy. I think this argument is wrong as a matter of principle and as a matter of politics, for reasons I've explained here before.
But whatever you think of gay marriage, one distinction between recognizing gay marriages and recognizing polygamous marriages should be clear. Recognizing gay marriages will involve primarily technical and word changes to existing marriage laws. For example, sex-specific references in statutes to "male" and "female" or to "husband" and "wife" would need to be changed to account for the fact that same-sex marriages are now legal. This itself will greatly trouble many people who believe something significant is lost in the elimination of that statutory language, but as a matter of draftsmanship it's not troubling. A couple of fairly minor substantive issues will have to be addressed as well (e.g., what do we do about the presumption of paternity in the case of a same-sex marriage?). Gay marriage will be an important change in policy. But it will not be all that big a change in statutory law itself because marriage will remain dyadic and existing rules presume a dyad. Massachusetts nicely illustrates the comparative ease with which the legal change can be made; literally nothing has had to change in state marriage law except that same-sex partners may now marry.
Contrast that with polygamy. Recognizing multiple-partner marriages would not only represent a big change in marriage policy, but as this post nicely explains in more detail, it would involve a very significant rewriting of marriage-related statutes. We could not simply say, "OK, now you may marry as many partners as you like," without having to give a great deal of forethought to how this change will affect the existing statutory attributes of marriage. The issues will not simply be technical or a matter of draftsmanship. Existing rules for property, inheritance, child custody, health-care decisions, financial responsibilities and rights, and divorce, will all have to be adjusted -- and in some cases adjusted dramatically -- to account for the presence of multiple stakeholders and decisionmakers. That's just a short and very general list. None of these policy complications apply to the recognition of dyadic same-sex marriages.
We could address all of these technical and substantive issues if we thought recognizing polygamous unions was, on balance, a good idea. But the fact that we'll have to address them, and might have to change some of the rules even for dyadic marriages as a consequence, must be counted as a significant cost against polygamy. By contrast, there's no analogous cost to recognizing same-sex marriages. And what's more noteworthy for the slippery-slope argument is how little of our extensive reconsideration of marriage rules would depend on whether we first recognized same-sex marriages.