Utah may limit legal marriage to one man and one woman (or, if it chose, two people of the same sex). It also has the right to punish the abuses that may accompany polygamy, such as rape, incest and welfare fraud. But it's hard to see where it gets the authority to dictate what words individuals may use for their relationships.
If Fred and I want to say we're cousins -- assuming we're not trying to defraud someone -- no prosecutor will bother us. If I refer to Sally as my aunt, despite the lack of family ties, the law is majestically indifferent. When Brown and his boos present themselves as husband and wives, though, he's applying for a prison cell.
In challenging the law, they can cite implicit support from the Supreme Court. In a 2003 decision striking down a Texas ban on homosexual sodomy, Justice Anthony Kennedy granted a wide berth to intimate relationships.
Anti-sodomy laws are unconstitutional, Kennedy said, because they interfere with "the most private human conduct, sexual behavior, and in the most private of places, the home," in order "to control a personal relationship that, whether or not entitled to formal recognition in the law, is within the liberty of persons to choose without being punished as criminals."
The same logic applies perfectly to Brown's plural "marriage." The state presumes to dictate how he represents his relations with consenting adult women, imposing harsh penalties if he does not comply.
If Brown wants to live with five women and call them his girlfriends, his shorties, his harem, the Seattle Storm or the 101st Airborne, it is of no earthly concern to the rest of us. And if he wants to call them his wives, the state of Utah should say, "Knock yourself out, dude." That, or nothing.