My co-blogger Todd Zywicki asks a very good question:

So the question is, if you get rid of the "man-woman" prong as largely arbitrary, why does this not lead to getting rid of the "one-one" prong as well? It seems like the new line is just as arbitrary as the old one.

The possible slippery slope from recognizing same-sex marriage to recognizing plural marriages has been one of the most common objections to same-sex marriage. (It is one of the objections raised in Justice Baxter's dissent in In re Marriage Cases, the California marriage decision.) It is among the most discussed sub-topics in the larger debate about gay marriage. While some gay-marriage advocates support recognition of plural unions, most others oppose it or are ambivalent about it.

Legislatures, Todd points out, do not have to articulate reasons why they refuse to move further than they have chosen to do so. So a legislature could authorize same-sex marriage, as the California legislature has twice done, without having to justify why it is not opening up the institution to polyamorous relationships. A court following the convention of rendering its results through judicial decision is expected to offer some potential limiting principle for why it won't go further.

Here is all the California Supreme Court had to say about why its decision would not necessarily lead it to declare a constitutional right to polygamy:

We emphasize that our conclusion that the constitutional right to marry properly must be interpreted to apply to gay individuals and gay couples does not mean that this constitutional right similarly must be understood to extend to polygamous or incestuous relationships. Past judicial decisions explain why our nation's culture has considered the latter types of relationships inimical to the mutually supportive and healthy family relationships promoted by the constitutional right to marry. (See, e.g., Reynolds v. United States (1878) 98 U.S. 145, 165-166; Davis v. Beason (1890) 133 U.S. 333, 341; People v. Scott (2007) 157 Cal.App.4th 189, 192-194; State v. Freeman (Ohio Ct.App. 2003) 801 N.E.2d 906, 909; Smith v. State (Tenn.Crim.App. 1999) 6 S.W.3d 512, 518-520.) Although the historic disparagement of and discrimination against gay individuals and gay couples clearly is no longer constitutionally permissible, the state continues to have a strong and adequate justification for refusing to officially sanction polygamous or incestuous relationships because of their potentially detrimental effect on a sound family environment. (Accord, e.g., Potter v. Murray City (C.D. Utah 1984) 585 F.Supp. 1126, 1137-1140, affd. (10th Cir. 1985) 760 F.2d 1065, 1068-1071, cert. den. (1985) 474 U.S. 849; People v. Scott, supra, 157 Cal.App.4th 189, 193-194.) Thus, our conclusion that it is improper to interpret the state constitutional right to marry as inapplicable to gay individuals or couples does not affect the constitutional validity of the existing legal prohibitions against polygamy and the marriage of close relatives.

In re Marriage Cases, slip op. at n. 52, 79-80.

This is unsatisfying as an answer to Todd's question. There isn't much meat in this footnote, except the citation to some hoary opinions from the Nineteenth Century. There is nothing like an articulated and non-arbitrary principle distinguishing same-sex marriage from plural marriage. In fact, resting on unadorned and unspecified societal fears that plural marriages have a "potentially detrimental effect on a sound family environment" sounds a lot like the kind of objections long made to recognizing same-sex unions — objections the court refuted at length in its opinion.

I am not faulting the California Supreme Court for failing to fully theorize now a distinction it might be required to make years hence. But I am pointing out that if we are looking for the kind of non-arbitrary principle that will prevent a logical slide to polygamy we will not likely find it so far in marriage decisions.

So is there an answer to Todd? I attempted an answer in a post here in 2005. I have also responded to the polygamy fears in shorter form in columns here and here. Jon Rauch has written about the unique problems created by polygamy here. Philosophy professor John Corvino has written about the distinction here. Many others have written on the topic.

I won't recapitulate these arguments or attempt anything like a comprehensive answer. Instead, here's a Cliff Note's version of some considerations that make proposals for polygamy different in principle from proposals for same-sex marriage:

*There is nothing in principle that necessarily leads from the recognition of a new type of monogamous union (same-sex unions) to the recognition of polygamous unions. Consider the recognition of inter-racial marriage (a type of monogamous union), which reversed long-standing legal bans on miscegenation and departed from deep cultural disapproval of it dating to colonial times and before. Many warned that reversing miscegenation bans would lead to polygamy, but it did not. To the objection that dyadic inter-racial unions would lead to polygamy, the proper response then was, "Why would it?" One response to the fear that dyadic same-sex unions will lead to a polygamy slippery now is, "Why would it?" Opening marriage to one change because the change seems justified does not mean that opening marriage to every change is justified. Every proposal for reform rises or falls on its own merits. Gay marriage advocates have made extensive (and contested) arguments about why it would benefit individuals and society. It is up to polygamy advocates to do the same.

*From a Burkean/Hayekian perspective, it's relevant that polygamy has been historically tried and rejected in many human societies. We do not write on a blank slate when it comes to polygamy. Lessons have been learned from this experience and those lessons have led us away from polygamy in the West, in part because polygamy as practiced has been seen as inconsistent with liberal values, individualism, and sex equality. SSM has not been tried and rejected and is not inconsistent with, indeed arises from, Western values of liberalism, individualism, and sex equality. While the burden is on gay marriage advocates to show why we should try it, I think actual historical experience with polygamy suggests that the burden on polygamy advocates is much heavier.

*Plural unions have historically most often taken the form of one man having many wives. It seems likely in practice it would take that form in the future. This raises many concerns different from those raised by same-sex marriage, including the greater potential for abuse of women and children. These same concerns do not arise with SSM, which should improve the lot of women and children in gay families (if SSM advocates are right about the benefits, a contestable but separate point).

*Polygamy will likely mean that marital opportunities will diminish for some men, since a few men who are very wealthy or otherwise attractive as mates will have many wives. This constricts the marriage market for less desirable men, which leaves some with no mates at all or delays their marriages as compared to their opportunities in a non-polygamous society. And unmarried men present all kinds of difficulties for societies. By contrast, SSM will mean that meaningful marital opportunities will be available for gay persons. More people will be married. Thus, SSM expands marriage opportunities while polygamy contracts them.

*With polygamy, many basic rules of marriage will have to be changed. For example: if the husband dies intestate, who inherits? How are death benefits split? How are child custody disputes decided if a partner wants to divorce the group? If the husband exits, do the wives remain married to each other? On and on. We could craft answers to these questions, but it will involve a dramatic retooling of marriage as a two-person institution. None of these issues arise with SSM; aside from a few technical matters, the marriage rules remain the same. As a legal matter, SSM involves changes in the wording of statutes that specify "husbands" and "wives" and little more. The basic legal design of marriage as a dyadic institution, embedded in literally hundreds of ways in state and federal law, remains untouched.

Perhaps none of this is conclusive against polygamy nor do I offer it as such. I am sure polygamy advocates have responses to these and other concerns about it. But I do think it suggests that SSM and polygamy present quite different questions of history, experience, logic, and public policy such that we are entitled to treat them as separate issues. We may, despite the concerns and the historical trend against polygamy, one day accept it. But the debate about accepting it will not, I think, turn on whether we have first accepted gay marriage.