Why Not Polygamy?
Rob Tisinai attempts to tackle the issue in the course of rebutting Robert George's laughably fallacy-ridden attempt to rationally justify heterosexual-only marriage. Tisinai is absolutely right that George fails utterly to show that his preferred conjugal view of marriage can justify monogamy. But I disagree that the alternative companionate model advocated by Tisinai gives any real justification for monogamy* either. And -- to lay my cards on the table early -- I infer from the failure of such pro-SSM-anti-polygamy arguments that reason compels us to give legal recognition to marriages on equal terms without regard to the number of people involved. Tisinai cites Jonathan Rauch as encapsulating his view of the justification for polygamy:
If marriage has any meaning at all, it is that when you collapse from a stroke, there will be another person whose "job" is to drop everything and come to your aid. ... No group could make such a commitment in quite the same way, because of a free-rider problem. If I were to marry three or four people, the pool of potential caregivers would be larger, but the situation would, perversely; make all of them less reliable ...
Concern over freeloading and proper distribution of responsibilities is certainly an issue that polygamous groups would need to consider. It may very well be a reason for someone to decide that they personally do not wish to have multiple partners. But I don't see how it's a reason to ban polygamy. Monogamous couples can encounter plenty of free riding too (what married couple has never fought over whose turn it is to do the dishes?).
Moreover, free riding is hardly the only possible threat to having a spouse there for you in a time of distress. I am my wife's only husband, but I might flake out on her because I'm busy with work, because I've become a jerk, or because I'm more concerned with helping a friend at that moment. All of these possible interferences are things that a smart person will weigh in deciding whether to marry someone. But that's their responsibility to mull over and their risk if they marry someone who, say, prioritizes their career over their spouse. The same seems to go for the free rider problem in polygamy.
This issue has been dealt with already in other areas of law where multiple people share a responsibility. It's called "joint and several liability." In a polygamous situation, each spouse is fully responsible for ensuring that the person in distress is taken care of, either by doing the care themselves or ensuring that someone else is doing it. (Note that the kind of outsourcing contemplated in that last clause is hardly alien to monogamy -- recently when I was in some distress while my wife was out of town, we both considered it the obvious best choice for me to seek comfort from some friends rather than for her to cancel the dream job interview she had the next morning in order to rush back to be by my side.)
Tisinai raises the issue of children as rivals for a spouse's primary responsibility, but I think it's even more instructive to look at the child-parent relationship on its own. After all, one of the reasons we work so hard to ensure that children are raised by parents, rather than dumping them into collective nurseries a la Brave New World, is that it's good for a child to have someone who has a special, primary responsibility for ensuring that child's welfare. A parent can be attentive to, and on call for, a child in a way that an orphanage supervisor can't. And yet we also take it as uncontroversial that giving a child two parents is, all else being equal, better than just one. We don't fear that having two moms will lead to each of them trying to free ride. We condemn deadbeat dads, but never infer from that that single parenthood is better for everyone.
From the way he talks about the advantages of monogamy, it's clear that the pragmatic concern with free riding is not the main thing that motivates Tisinai. Rather, he seems quite drawn to the comfort of knowing that he and another person are number one in each other's eyes. I can sympathize with that. I like knowing that my wife and I are each each other's primary responsibility, and even if it were legal I doubt I could accept another partner being on equal footing. But I also realize that that is not a universal feeling (after all, if it were, we wouldn't need a law against polygamy!). The real question is, if someone believes that they can make the deep commitment of marriage work with another person, what public policy good is served by forbidding them from trying? (Or rather, denying them any legal standing for their attempt, since we're not arresting people for living in de facto polygamous arrangements.)
Perhaps it's not strategic for the cause to admit this, but if someone tells me that same-sex marriage will inevitably lead to polygamy, my response is "great, we'll kill two birds with one stone then."
*In this post we're talking specifically about a defense of monogamy, that is, legal recognition of a single spouse. George conflates legal monogamy with total sexual exclusivity. Tisinai recognizes that there are other forms of non-exclusivity that would not be ruled out by his main argument -- you could have other sexual partners, or even "secondary" dating relationships, without running afoul of his argument for having a single "primary" spouse. Tisinai does attempt to rule out such other forms of non-exclusivity, but his argument on that count consists of denying that more casual sex is possible (or at least that the risk of failure is too high to make it acceptable). This argument is clearly contrary to the evidence, but I don't want to make this post more rambling by trying to address it in detail.