The Fundamentalist Christian Right(tm) is fond of saying that if marriage was allowed for gays than its just a matter of time that polygamists would demand it too. There are many reasons why this is complete idiocy but this article in the New York times points out an important issue with why polygamy doesn't work. Economics.
Over the last six years, hundreds of teenage boys have been expelled or felt compelled to leave the polygamous settlement that straddles Colorado City, Ariz., and Hildale, Utah.Emphasis mine.
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“In part it’s an issue of control,” Mr. Murphy said of the harsh rules. But underlying the expulsions, he added, is a mathematical reality. “If you’re going to have plural marriage, you need fewer men,” he said.