One thing you sometimes hear from the anti-gay marriage folks is that if we ‘change the definition’ of marriage to allow for same-sex marriages then we will have to allow polygamist and incestuous marriages, perhaps even people marrying animals (I actually think we should seperate marriage and civil unions but I ignore that issue in this post) . To start with the easiest first, animals. Whatever else marriage is it involves promise-making between two agents capable of making promises. This automatically precludes the possibility of marrying animals (in the cultures that allow people-animal marriage there is usually the background assumption of reincarnation thus the animal is thought of as something that can make a promise).
But what about incest? What is wrong with incest? This is a difficult question to answer. By incest I mean consenting sexual relations between related individuals (this rules out child molestation). But how related? It is legal to marry a 2nd or 3rd cousin. Is this incestuous marriage? Well, what about a brother/sister or father/daughter marriage? Should these be allowed? Arguably not. IF, as some people think, marriage is generally intended to encourage and support child rearing then the state has an interest in forbidding these kinds of marriages. Incestuous sexual relations of this sort almost always lead to children that are genetically damaged. In fact this is probably the reason that we are disposed to find this kind of behavior so revolting. But if the incest is really between consenting adults and precautions are taken against pregnancy then we should not automatically rule this as immoral. I think that, in a very, very, small number of cases it might turn out that this was allowable, but we still would not have to approve of incestuous marriages.
So, what about polygamy? If we endorse gay marriage are we committed to allowing polygamist marriages? First we must ask if there is anything wrong with these kinds of marriages? Martha Nussbaum has recently defended polygamy. I must confess that I find her views reasonable. If all parties consent, and if women are allowed to have multiple husbands (polyandry, apparently) as well as men having multiple wives, it is hard for me to see why we should care about this. Why does it matter if the Big Love people do what they do. True this isn’t for everyone, but no one is asking for it to be.
But even so, are we committed to allowing polygamy? I am not convinced that we are.
UPDATE: See the comments section of Nussbaum’s post for some interesting arguments against polygamy from Thom Brooks.