A somewhat better argument than the first two I've addressed today (the definitional argument and the contagious-promiscuity argument) is the polygamy slippery-slope argument.

Slippery-slope arguments offer a parade of horribles that might be brought about by gay marriage, but they always have this form: "If we allow gay marriage, we'll end up with [policy X], and that would unquestionably be bad." The usual bad destination claimed to await us after gay marriage is polygamy. But one occasionally hears that gay marriage will also bring incestuous marriages, bestial marriages, etc. Here I will consider only the polygamy variant of the slippery-slope argument because it's by far the most common, but much of what I have to say would apply to other slippery slopes.

Theoretically, slippery slopes can be initiated in one of two ways: (1) the logic of the proposed step (gay marriage) entails a slide down the slope; or (2) the politics of the proposed step, e.g., in terms of the way in which it might liberalize public attitudes about further reform, risks a slide down the slope. In reality, however, if there is no political momentum for a reform, logic alone will not likely produce a slide.

If gay marriage led to polygamy that might please some people, but it would not be welcome news to the traditionalist.

1. The political slide to polygamy.

The political slide that might be initiated by gay marriage has been addressed in some detail by Eugene in Same-Sex Marriage and Slippery Slopes, 34 Hofstra L. Rev. ___ (forthcoming 2006), available at http://www.hofstra.edu/PDF/law_lawrev_volokh_vol33no4.pdf. He concludes that the political prospects for polygamy, after gay marriage is adopted, will be "lousy." The political right will not support it. And the political left will likely not be supportive, either, for several reasons he lists. You can find a few leftish supporters of polyamorous marriages, especially among academics. But academics have many esoteric causes.

If neither the right nor the left will line up behind you, your prospects of success are very dim. So no matter what we do about gay marriage, polygamy will not arrive, especially in the West, where liberal individualism, sex equality, and the loss of polygamy's own religious adherents, all combine to make it a very rare and dying practice.

2. The logical slide to polygamy.

That leaves the supposed logical slide to polygamy, which is almost always the slide envisioned by gay-marriage opponents.

What is the necessary logic behind gay marriage that will leave us no principled choice but to accept polygamy? To be sure, one could make (and some have made) arguments for gay marriage that seem very open-ended.

One possible principle uniting gay marriage and polygamous marriage is that gay marriage, like polygamous marriage, extends marriage beyond partners who may procreate as partners. If there is no necessary link between marriage and procreation then maybe we will have to recognize all arrangements, like polygamous marriages, which cannot form a child from all of the partners.

The notion that gay marriage fundamentally severs the link between procreation and marriage, and thus leads to polygamy, founders on the same logical and experiential shoals as does the procreation argument (which I'll discuss tomorrow). Briefly, procreation is already not a requirement of marriage. Sterile opposite-sex couples have already taken that step down the slope for us, yet we are no closer to polygamy.

A second possible uniting principle is that gay marriage necessarily makes marriage a private affair, catering to the wants and needs of private adult citizens, not an institution with a profound public purpose, like ensuring the raising of the next generation. If marriage is a private matter, the argument goes, then the state has no business regulating entry to it, so polygamous groups cannot be denied marriage.

This supposed uniting principle misconceives the argument for gay marriage, which, as I have outlined it, is not necessarily based solely on augmenting the private happiness of two adults. Further, using marriage to recognize adult love is a step down the slope already taken by straight couples. Like it or not, many people in the West already understand marriage as companionate; they don't need gay marriage to reach that conclusion. So even if gay marriage were justified solely by the love same-sex partners have for one another, recognizing such relationships would be more analogous to taking a step to one side on a slope already partially descended, not an additional step down the slope.

I think this should be enough to reject the idea of a logical slide. There's just no good reason to think that recognizing a new form of monogamous marriage logically entails recognizing polygamous marriage.

But for those still uncertain, let me take the argument one step further. Gay marriage and polygamy are not only not united by any single common principle necessary to the argument for gay marriage, but for the traditionalist, the affirmative arguments for them are quite distinct.

Here's why. Any proposal for the expansion of marriage must be good for both individualistic reasons and communitarian reasons. Gay marriage meets both criteria, as I have shown. While I don't want to offer any definitive conclusions about polygamy here, I think the case for polygamous marriage is distinguishable (and weaker) on both counts, especially the second.

On the first issue - - the individualistic benefits - - there are good reasons to doubt whether polygamous marriage would produce the same degree of caretaking and social benefits gay marriage would produce.

While multi-partner marriages might benefit the partners involved, the much greater potential for jealousy and rivalry among the partners make for a potentially more volatile arrangement than a two-person marriage, reducing the expected caretaking benefits to its participants. In a multi-partner marriage, it may also be unclear who has primary caretaking responsibility if a partner becomes sick or injured; there is no such uncertainty in a two-person marriage. While we have good evidence that children do well when raised by two parents, including same-sex couples, we have no evidence they do well when raised in communal living arrangements.

The expected social benefit from polygamy (e.g., the reinforcement of the marriage by others) should also be smaller if, as I argued above, public resistance to polygamy will be large and unyielding.

On the second issue — the communitarian benefits — the differences between gay marriage and polygamous marriage are potentially more pronounced. There are communitarian benefits to gay marriage; there may well be serious communitarian harms to polygamy.

Since multi-partner marriages have almost always taken the form of one man having many wives, recognizing them presents special risks of exploitation and subordination of women, which is inconsistent with our society's commitment to sex equality. There is no comparable concern raised by gay marriage.

In human history, polygamy has correlated strongly with societies that were illiberal and undemocratic. Gay marriage is arising in the most liberal societies, characterized by representative democracy, widespread franchise, and universal education.

Is this correlation relevant? Why does it exist? Several explanations are possible, but two are most important here. First, modern liberal societies have emphasized values like individualism and sex equality that seem inconsistent with polygamy as it has been practiced. Gay marriage, by contrast, is fully consistent with these values.

Second, polygamy takes many more women than men out of the marriage pool. This leaves heterosexual men with fewer marriage opportunities. Unattached men with poor marital prospects destabilize societies, and large numbers of such men in a society require strong mechanisms of state control to rein them in. Gay marriage helps ensure marriageable partners for everyone; polygamy does the opposite, with potentially anti-liberal, undemocratic, and socially destabilizing consequences. (The communitarian harm from polygamy might be small because few people will be polygamous, but a small harm is still a harm.)

Whatever the strength of a Burkean case against gay marriage (and I'll get to that Friday), the Burkean case against polygamy is much stronger. Polygamy, unlike gay marriage, has been tried and rejected. Many human societies have practiced it at one time or another and almost all have abandoned it; gay marriage, by contrast, has never been tried and rejected.

Perhaps none of these considerations is decisive against the recognition of polygamous marriages, nor do they need to be in order to make the point. This discussion shows that gay marriage and polygamous marriage present very different issues of history, data, logic, and experience. And nothing in this complex discussion of history, data, logic, or experience turns on whether gay marriages have previously been recognized. Gay marriage and polygamous marriage should each be evaluated on its own merits, not treated as if one is a necessary extension of the other.

Finally, it should be said that slippery-slope arguments about marriage have a certain Chicken Little quality about them. The ominous slide to polygamy has been a favorite trope. For example, the same polygamy red flags were raised about interracial marriage. In the Nineteenth Century, the Tennessee Supreme Court warned that the recognition of such marriages would lead to "the father living with his daughter . . . in wedlock" and "the Turk . . . establish[ing] his harem at the doors of the capitol." State v. Bell, 66 Tenn. 9 (Tenn. 1872).

This is not to say that warnings about slippery slopes, even about slippery slopes in marriage reform, have never proven true. But it is to say that nothing in the traditionalist case for gay marriage brings us any closer to the harem than we were when the Tennessee Supreme Court warned us about it more than a century ago. And if gay marriage is ever accepted in America, I believe it will be on the basis of something like the traditionalist grounds I have offered. That is, it will be accepted when Americans have become convinced that gay marriage is a good idea for traditionalizing individualistic and communitarian reasons.

Tomorrow, I'll deal with the procreation argument in two parts: one post on the standard version and one post on Maggie's more subtle version.