Actually, Utah WILL prosecute anyone engaging in LEGAL bigamy (i.e. applying for a marriage license while married). It’s a 3rd-degree Felony.
Currently, Utah has a law which makes it a crime to engage in UNOFFICIAL bigamy – that is, more than two adults living together in a relationship, but without having had a marriage license (i.e. no LEGAL bigamy going on). There is a current lawsuit challenging this concept, and, as of Jan 2014, the plaintiffs have won, overturning this law. The case is ongoing, however, and on appeal.