I haven’t posted much about Same-Sex Marriage (SSM) in the last year or so, but an interesting post by Dale Carpenter caught my eye. He responds to the standard slippery slope argument against SSM, that recognizing SSM will require the government to recognize polygamy because no principled distinction between the two can be drawn. His counterargument is that allowing SSM will require only minor changes to the existing marriage codes, but legalizing polygamy will require a complete overhaul of existing laws. Changing those laws imposes a significant cost than SSM does not generate. As Gabriel of Galios once put it, SSM only requires changing who can play the game, but polygamy requires changing the rules of the game itself.
The analogy is not completely accurate. Many of our family and custody laws are based on common law theories that assumed specific gender roles for the parties. For example, custody laws often favor women, who are assumed to be better parents. Also, divorce laws require maintenance and child support are underpinned by the idea that women would stay at home with the children and need to be supported to do so (although this scheme has been partly changed to allow more equality). SSM would certainly require redoing some laws (or at least the assumptions behind them), and that would be a cost as well.
Nevertheless Carpenter’s argument is basically correct. But that doesn’t mean his counterargument completely deflects the polygamy/SSM analogy. Legalizing any new form of marriage would require a weighing of distinct benefits and costs that apply only to that form of marriage. For example incest might not require as an extreme overhaul of marriage laws (although it would certainly affect inheritance laws), but would generate other costs (e.g., the fear of forced marriages, deformities in offspring, etc.).
The argument that Carpenter is responding to is not (or shouldn’t be) that polygamy and SSM are exactly the same and if we recognize one, we must recognize the other. The argument is that once society begins to change the fundamental definition of marriage in one case, it is required to review other forms of marriage and embark on a cost-benefit analysis for the other forms. It cannot just deflect arguments to change marriage by saying “that’s how our society understands marriage.” Legalizing SSM certainly weakens the arguments against polygamy and that’s a cost to legalizing SSM. While the benefits of legalizing SSM might still outweigh the costs, SSM advocates must recognize the costs to their proposal.