One of the more popular arguments against the legalization of homosexual marriage is that it will open the door to polygamous marriages, cross-species marriages, incestuous marriages, etc. The argument is often dismissed as a “slippery slope” that is unjustified.
I have argued in the past that properly construed, the argument isn’t a “slippery slope” argument at all. That is, society may in fact never legalize polygamous marriages. However, the society that acknowledges homosexual marriage will have no principle to fall back on to prevent polygamous marriages. Hadley Arkes, in his excellent essay “The Family and the Laws,” makes the same point:
We’ve also learned over the ages that the law teaches: If the law becomes open to the arrangements of mothers and sons, fathers and sons, marrying, we can expect that these arrangements will not stay rare and bizarre–that we will come to see far more of them. But whether we see more or less, the people who claim rights of same-sex marriage have to deal with this critical problem of their argument: that they have no ground in principle to deny any longer any of these other arrangements. They cannot explain why marriage should be confined to a couple, rather than the ensemble of three or four or more who claim to be intimate and loving. Some of us made this argument eight years ago during the hearings over the Defense of Marriage Act. And sure enough, as though sprung from the argument itself, we have now seent he advent of the “polyamorous,” the people who claim a right to be joined in marriage to the fuller range of people they are capable of loving. But if the notion of marriage comes to encompass the polyamorous or the polygamous, or the father and son, or the two brothers, the notion of marriage will have lost its coherence. And along with that coherence, it will have lost the most compelling ground of its explanation and defense, as something desirable, something we are justified in preserving.
If “marriage” is constituted by intimacy, regardless of the possibility of procreation, then there is no possible grounds on which any intimate relation could be barred from marriage. As Arkes goes on to point out, an erotic attachment wouldn’t even need be necessary: two sisters taking care of an elderly father might want to marry in order to take advantage of special provisions for insurance, joint tax returns, etc.
The question of homosexual marriage is not a question of who we should allow into the institution of matrimony. It is a question of whether marriage should exist as a meaningful idea at all, or whether we wish to remove the principle that undergirds it, a principle that demands the monogamous union of a man and a woman.