William Saletan, writing in Slate, doesn't think much of the argument that legalizing gay marriage will make polyamory (polygamy and group "love") inevitable. He states that human jealousy insures that such unions don't last, do not work, and that there will not be a large constituency for them.
His premises may all be correct, it may be true, for instance, that jealousy is an ineradicable human trait, but his conclusion that it would prevent people from wanting to form marriages between multiple partners simply doesn't follow. Just because certain kinds of unions are inherently unstable doesn't mean that people won't want to try them. It only takes a few people who wish to push the envelope, even if they themselves are not interested in a life-long commitment, to challenge the law. And when they do there will be no logical or legal justification for denying them the right to try to make the unorthodox arrangement work.
Another human trait is that people have a tendency to pull down every stop sign along the highway to total freedom, especially sexual freedom. People can be found who will try to do whatever they think they can do. Once the gender of those entering into marriage is no longer legally relevant there will be no justification for arbitrarily saying that the number of people entering into a marriage is legally relevant. Mr. Saletan is simply naive if he thinks that the human predisposition toward jealousy will insure that the law will not give way to logic.