Utah's law is unique in that a person can be found guilty ... for cohabiting with another adult in a marriage-like relationship when they are already legally married to someone else.
In other words, Utah's law criminalizes adultery of the cohabitation variety.
That hardly makes it unique, however, because quite a few states still have laws on their books that criminalize adultery; those laws, of course, are rarely if ever enforced. For example, in NY it is still technically a misdemeanor. In some other states, e.g., MA, it's a felony.
That being said, the court's decision viz. the Fourteenth Amendment is probably sound on the basis of the Supreme Court's decision in Lawrence v. Texas, 539 U.S. 558 (2003)
, in which it held that intimate consensual sexual conduct between adults was part of the liberty protected by substantive due process under the Fourteenth Amendment. That Lawrence
dealt with consensual sex between two adults of the same gender should be irrelevant to the core holding in Lawrence
, so the decision in this case would most likely be upheld by the Supreme Court for the reasons it articulated in Lawrence
for invalidating state laws that criminalized sodomy in private between consenting adults of the same gender.
The First Amendment/free exercise part of this decision is on much more questionable ground.