Not really, they are allowed even if it is technically illegal.
" Today, over 14 million members of The Church of Jesus Christ of Latter-day Saints (the Mormons) are strictly monogamist, and members who are known to practice polygamy are excommunicated. Still, the practice of plural marriage continues among tens of thousands of members of various fundamentalist splinter groups long disassociated from the main body of the church, such as the Apostolic United Brethren (AUB) or the Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS). These polygamist sects are generally located in Utah, Arizona, Texas, and other parts of the Western United States, Canada, and Mexico. Even though polygamy is generally illegal in all 50 states and in all three countries, practitioners are almost never prosecuted unless there is evidence of abuse, statutory rape, welfare fraud, or tax evasion."