US advocacy groups for polygamy and individual liberties have hailed a federal judge's ruling that key parts of Utah's polygamy laws are unconstitutional, saying it will remove the threat of arrest for those families.
The main star of TV reality series Sister Wives in particular has rejoiced at the ruling after having problems with Utah's strict laws.
Polygamy is banned in all 50 US states, but in Utah co-inhabiting with a woman in a situation that was like marriage while being married was also criminalised.
US District Judge Clark Waddoups said in the decision handed down on Friday that a provision in Utah law forbidding cohabitation with another person violated the First Amendment of the US Constitution, which guarantees basic rights, including freedom of religion.
The ruling is a victory for Kody Brown and his four wives who star in the hit TLC cable TV reality show Sister Wives and other fundamentalist Mormons who believe polygamy brings exaltation in heaven.
The Brown family filed their lawsuit in July 2011 and fled Utah for Las Vegas last year under the threat of prosecution.
Anne Wilde of Salt Lake City, co-founder of the polygamy advocacy group Principle Voices, said polygamous families have lived under the threat of arrest for decades and no longer have to worry about being charged with a felony.
There are an estimated 38,000 fundamentalist Mormons who practice or believe in polygamy, most living in Utah and other Western states, said Wilde, who was a plural wife for 33 years until her husband died.
"Now that we're no longer felons, that's a huge relief," she told The Associated Press.
"They no longer have to be afraid that someone will knock at their door and take away their kids. This decision will hopefully take away the stigma of living a principle that's a strongly held religious belief."
Connor Boyack, president of the Libertas Institute says the ruling represents "a new beginning and an important invalidation of an unjust law".
He said that while child brides and abuse must be appropriately prosecuted, consenting adults in a plural relationship should not face penalties.
Polygamy is a legacy of the early teachings of the Mormon church. The mainstream church abandoned polygamy in 1890 as Utah moved toward US statehood. Today, it strictly prohibits the practise.
Under Waddoups' ruling, bigamy remains illegal in Utah only in the literal sense, such as when someone fraudulently acquires more than one marriage licence.
Utah officials had sought to have the lawsuit thrown out. They argued the Browns couldn't challenge the bigamy law because they hadn't been charged. State attorneys later argued for dismissal because a prosecutor had pledged not to prosecute them for bigamy.
Kody Brown said his family is "humbled and grateful" for the ruling.