It should be noted that polygamy is against the law in Utah. The Utah constitution in fact requires that polygamy shall never be legalized in Utah. This is a result of a law made in the late 1800s illegalizing polygamy in all US territories, and establishing the permanent illegalization of polygamy as a requirement to gaining statehood.
Because of the law against polygamy, religious groups in Utah that promote polygamy can be subject to rather severe penalties.
Note that the US Constitution reserves the legal competence for regulating marriage to the several states, although the national government is made competent to make laws concerning family welfare in territories.
There are apparently individuals who practice polygamy in Utah. Mormons who practice or promote polygamy are generally excommunicated, but the state does not usually seek polygamists out to throw them in jail unless other crimes, such as wife abuse or child abuse, are committed.