As for polygamy, I think Utah gave it up as a condition of statehood. I could be wrong about that, but I'm pretty sure that they weren't getting in otherwise.
It is lawful. Watch and see. We Republicans are going to take back our country from the liberals and judges.
See http://www.religioustolerance.org/lds_poly.htm
In actions reminiscent of the "same-sex marriage" debate which started in the late 1990's, the Mormon practice of polygyny was countered by the federal Morrill Act of 1862 and the Edmunds Act of 1882, both of which prohibited the practice within the U.S. The Church claimed that the federal government had no jurisdiction to regulate marriage and other internal church practices. They also claimed that the act was a violation of Mromon's First Amendment rights. In 1879, the US Supreme Court declared that the Morrill Act was constitutional, that the government had a right to enforce marital standards, and that polygyny was a barbarous practice.
The federal government canceled the citizenship rights of polygamous Mormons. They were no longer allowed to vote, run for public office, or serve on a jury. Eventually, the government disincorporated the Church and began to confiscate its assets. In 1890, the Supreme Court determined that the government could take away citizenships from all members of the church.
That would be correct and the feds gave Utah a choice, accept the federal definition of marriage as the union of one man and one woman or join Mexico.
That is the precedent for the federal governments defining marriage while states regualte it. All the rest of the states rights bs is simply that bs.
Marriage is and has been defined by the feds as the union of one man and one woman.