There are constitutional prohibitions in both the Utah and Arizona state constitutions. And because the federal government didn't trust the people of Utah, the Utah constitution's anti-polygamy provision specifically prohibits the people of Utah from changing it without the consent of the United States. That was part of the deal for Utah to gain statehood.
All paper. Forgive me for not being impressed with such safeguards.
Utah will override any state constitutional provisions if other states start allowing it and possibly would do so before to be the first state to legalize it.