David Link, an attorney who has worked in the California legislature for eight years and has written for Reason and the L.A. Times, has followed the recent exchange among Professor Robert George, Jon Rauch, Maggie Gallagher, and me over gay marriage and polygamy. He has emailed me his thoughts on some practical differences between recognizing dyadic same-sex marriages and recognizing polygamous/polyamorous marriages.

Multiple partner marriages, he argues, would raise many issues simply not present in gay marriages:

Some recent events related to polygamy have brought the slippery slope argument back into prominence for gay marriage advocates. See reactions to Robert George here. The argument has, as a premise, that same-sex marriage is enough like polygamy that legislatures or courts could not distinguish them; if the first is approved, the second would likely follow. In Professor George's words, there is no "principled" or "serious" argument that could accept same-sex marriage but not polygamy.

But there are differences between same-sex marriage and polygamy that would make it perfectly sensible for society -- whether acting through a legislature, a vote of the people, or a court — to draw a line that includes same-sex marriage, but excludes polygamy.

The key difference can be found by asking a fairly simple question that gets very little focus in the current debate: in a polygamous marriage, who is married to whom?

Since polygamy is illegal in America, we seldom have reason to think about such an obvious question. But it's at the heart of the reason some of us would be perfectly comfortable saying that polygamy is so different from same-sex marriage that the one could fit into our understanding of marriage, while the other does not.

The difference comes down to arithmetic. Same-sex marriages have the same dyadic structure that all heterosexual marriages now have. Each partner is married to the other, and only to the other. Their rights and obligations to one another, to any children they may have, and to any third parties who might have some interest in the relationship, such as banks, creditors, parties to contracts, etc., are usually quite clear.

That's not true with polygamy.

In the dominant form of polygamy, where one man is married to several wives, he is, in some way, "married" to each one of the wives individually. But the exact boundaries of such relationships are unclear, and we have no modern experience to know how far they might extend.

But what about the relationships of the wives to one another? Are they similarly "married" to all the other wives in the marriage? Specifically, as a matter of public policy, are they legally married to one another the way a husband and wife are under current marriage law?

Stay with that question. If the answer is "yes," then if the husband died, would the wives continue to be married to each other? Why or why not? For those who find same-sex marriage objectionable, why wouldn't those relationships among the wives be same-sex marriages? In ancient cultures where women may have had fewer rights than men, such questions might never have come up. But they would be inevitable in today's world if polygamy were to be seriously debated.

And every question like these leads to others. Assume the husband is alive, but relationships with him sour. Could some or all of the wives divorce the husband, but continue to be married to one another? Could they divorce one another? Again, why or why not? And if the answer is "yes," how would that work? Who files what papers, naming whom? Would the various partners choose up sides in the ensuing divorce proceedings, and how would a court deal with that?

Another question related to divorce: Could an individual wife file for divorce of only herself, or would a divorce petition dissolve the entire marriage? What about if it's the husband who wants a divorce? Should the rule for him be different than the rule for the women -- i.e. could his successful petition for divorce dissolve the entire marriage, while a wife's successful petition only removed her from the marriage? Or consider the situation where one woman is married to several husbands -- or where several women are married to several husbands. Again, who would be able to divorce whom, and why? How would such actions affect other spouses?

And -- central to the present debate — what about the children? If the husband -- or one of the wives -- wanted out of a polygamous marriage, what would the rules be for who gets custody of the children -- and who is responsible for child support? Do the other wives have a claim to custody, along with their husband? What about child support payments? All the wives would almost certainly have some long-term relationship with children of each of them. Would it be good for the children to cut off those relationships because one wife wanted out of the marriage?

The questions related to divorce illustrate only the legal and policy problems within the marriage. But what about the critical question of how outside parties would be affected by polygamous marriages -- no small thing in the modern world. There are clear rules when a contractor signs up to remodel a married couple's kitchen about the couple's legal responsibility for payment. But what about a contractor remodeling a polygamous family's kitchen? If, as in "Big Love," each wife has her own house, and the one who gets the remodel can't or doesn't pay, can the contractor go after a wife with sounder finances? Again, why or why not?

The fact that we do not know the answers to these questions -- and thousands of others -- is at the core of why polygamy is dramatically different, as a matter of public policy, from same-sex marriage.

If anyone wants to argue in favor of polygamy -- and for the present such advocates still remain either imaginary or well out of the political mainstream -- they will have a lot more questions to answer than advocates for same-sex marriage do. That is because of a very simple reason. Same-sex marriage has the arithmetic on its side. It is mutual, binary, and fully capable of being subject of all existing laws related to marriage.

Polygamy would require a genuine rethinking of marriage. And its multiplicity truly does have the capacity to undermine marriage: psychologically, culturally and legally. In fact, polygamy offers exactly the kind of concrete danger to marriage as we know it that same-sex marriage opponents have only been able to insinuate. This difference between same-sex marriage and polygamy can serve as at least one sound basis to argue that same-sex marriage is consistent with marriage as we understand it in today's world, but polygamous marriage is not.

These issues could be addressed, special rules worked out to accommodate the many differences between dyadic and multiple-partner marriages, etc. But they are large and numerous enough to suggest that the slope is a lot more sticky than some people suppose.

UPDATE: A number of commenters have suggested that the problems David Link points to have been addressed in one way or another by societies that have practiced polygamy for thousands of years — and thus the problems are not insuperable. "Polygamy has been quite common in Islamic societies," notes one commenter, and surely we can learn from them. But this only exposes a much larger problem that separates the recognition of same-sex marriage from the recognition of polygamous marriage: sex equality. Marriage in the West has evolved over the past 150 years or so into an equal partnership, where both man and woman have more or less equal rights and responsibilities. The legal distinctions between men and women in marriage have been largely erased. In the traditionalist, pre-modern societies where polygamy flourished this norm of sex equality was simply not present. Gay marriage, by contrast, is fully consistent with our commitment to sex equality. In fact, it is partly an outgrowth of that commitment.

Making polygamy (and multiple-partner marriage generally) work in a modern society where sex-equality norms are strong would not be a simple matter of transferring the legal rules from these other, earlier societies to ours. It would require a great deal of adjustment, both to our marriage practices and to the historical practice of polygamy. Again, the point is not that these adjustments could not be made — with a whole lot of effort they could be, at some cost. The point is that having to make them separates the issue from dyadic same-sex marriage in a way that makes either the logical or political/ideological slide seem unlikely.

The practical issues Link raises are not the only reason there's no slippery slope from gay marriage to multiple-partner marriage, as I have written previously here and here, and Eugene has discussed in a very thorough law review article on the subject, but they are a part of it.