Congress only allowed Utah, and Idaho, and Arizona, and Oklahoma, into the Union if they would forever foreswear plural marriage.
This move is a direct challenge to the moral natural law basis of our free republic, in more ways than one.
Yes, these states had to ban polygamy to be allowed into the union.
A Supreme Court decision in 1879, I believe, upheld a federal law which banned polygamy in U.S. territories such as Utah. That decision said that it was not a violation of Mormon religious freedom to limit marriage to 1 man and 1 woman.