In Utah and other western states, polygamy is not unheard of. In fact, there are an estimated 30,000 polygamists living in the West. But the last prosecution of a polygamist in Utah was in the 1950s. It does not appear the state has been very zealous in enforcing its polygamy ban. If the state is going to enforce polygamy laws, it should do so evenhandedly and not merely go after the polygamists who aren't quiet about their lifestyle.
Critics claim polygamy fosters crimes such as child abuse and welfare fraud.
That may be true. However, that is no reason to ban the practice. Just as drug abuse might lead to theft and murder, that is no reason to ban drugs. Prosecute the crimes, not behavior that may or may not lead to that crime. The state should punish child abusers.