Here comes the campaign to legalize/normalize polygamy. As predicted, scoffing from advocates of "gay marriage" notwithstanding.
Here's a key paragraph that says it well:
The definition of marriage is plastic. Just like heterosexual marriage is no better or worse than homosexual marriage, marriage between two consenting adults is not inherently more or less “correct” than marriage among three (or four, or six) consenting adults. Though polygamists are a minority—a tiny minority, in fact—freedom has no value unless it extends to even the smallest and most marginalized groups among us. So let’s fight for marriage equality until it extends to every same-sex couple in the United States—and then let’s keep fighting. We’re not done yet.
That's actually a sound argument--given the premise, namely: that "heterosexual marriage is no better or worse than homosexual marriage." Of course, I don't believe that; but once that premise is established, then it follows that "marriage between two consenting adults is not inherently more or less 'correct' than marriage among three (or four, or six) consenting adults."
Indeed, I would point out that while the prohibition on marriage being same-sex can be rooted in nature--and not in any particular religion--the restriction of marriage to two, and only two, contracting parties is particular to some religions. Which raises a question: will advocates of polygamy argue that the restriction to two persons is cannot be justified on a non-religious--and hence, constitutional--basis? I'm not an attorney, but that approach would interest me if I were carrying that water.