[Retric]

From the article you linked.

“On December 13, 2013, a federal judge, spurred by the American Civil Liberties Union and other groups,[25] struck down the parts of Utah's bigamy law that criminalized cohabitation, while also acknowledging that the state may still enforce bans on having multiple marriage licenses.[26] The state of Utah appealed the decision, arguing that polygamist Kody Brown (whose relationships were documented in the show Sister Wives) lacked standing to bring his civil suit, since his county prosecutor, Jeff Buhman, had not followed through on any plan to prosecute the Brown family. The Tenth Circuit Court of Appeals (Denver) agreed with Utah and overturned the previous decision, thus effectively recriminalizing polygamy as a felony.[27]”

Of note, the state of Utah won the appeal because they didn’t bring action against the group rather than because the lower court’s decision was incorrect. Thus polygamy can remain illegal as long as nobody is ever prosecuted for it.

It also mentions: “Given that almost no polygamists bother to seek a second marriage license, the practice of forming a family with more than one spousal-styled relationship is very difficult to criminalize. In the majority of cases, the additional partner is considered a wife in the context of religious beliefs. Legally speaking, the practice is more akin to adultery.”