One of the arguments that people make when opposing gay marriage is that allowing gay marriage could lead to polygamy being legalized. It’s the classic “slippery slope” argument. While one case does not a trend make, three people were recently married in the Netherlands. Technically they just registered with the state as a civil union, but they still enjoy all the benefits of marriage. If nothing else, this should cause people to at least consider the polygamy objection rather than treating it as absurd. There are people in the world that would like to be a member of a polygamous relationship, and some of them will certainly come forward to request the right to marry if gay marriage is legalized. The question would just be “when”, not “if”.
Now I doubt that Americans would be very receptive to the idea of polygamous marriage. It would likely garner even less support than gay marriage. But that wouldn’t matter from a legal standpoint. If marriage could be redefined to include gay couples, why not include unions of three people? I have not seen any good reasons why one would be allowed but not the other. In fact, what I usually see is for the person defending gay marriage to simply declare that a marriage has to be between two people. That is no different than a gay marriage opponent stating that marriage has to be between a man and a woman. The polygamy objection seems to me to be a legitimate criticism of the proposal to redefine marriage. So gay marriage advocates need to develop arguments which defend gay marriage but exclude polygamy. If someone isn’t prepared to take that obejction seriously in light of the events in the Netherlands, then why should anyone take his arguments seriously?