A federal court declared a part of the Utah law against polygamy as unconstitutional so of course all of the religious right morons are saying "See we told you so, a slippery slope. Allow Gay Marriage and before you know it people will be marrying their dogs."
Which is of course total nonsense.
The judge's ruling had to do with the co-habitation part of the law. The basic question he asked was what's the difference between a married man living with his wife and two women who are not his wife and a man living with three women none of which are his wife?
The state blew it because there IS a fundamental difference. When the state issues a marriage license, it confers financial and legal benefits upon the two parties and it has a reasonable expectation that the couple will form a two adult relationship. If no marriage certificate has been issued, then the state confers no benefits and has no expectations.
The ruling does not legalize polygamy. You still cannot be legally married, with all of the rights, obligations and privileges that entails, to more than one person.