The landmark decision is seen as just a step in a legal odyssey that will likely see the Supreme Court of Canada decide whether the Charter of Rights and Freedoms protects the religious practice of multiple marriage.
Chief Justice Robert Bauman of the B.C. Supreme Court concluded there is no such right.
The Criminal Code section banning polygamy does infringe on the right to freedom of religion, Bauman concluded, but that was outweighed by the long list of harms polygamy inflicts on women and children, including sexual and physical abuse, child brides and the institutionalization of gender inequality.
He said the law is valid as long as it's not used to prosecute children.
"This case is essentially about harm. ... This includes harm to women, to children, to society and to the institution of monogamous marriage," wrote Bauman.
"Polygamy's harm to society includes the critical fact that a great many of its individual harms are not specific to any particular religious, cultural or regional context. They can be generalized and expected to occur wherever polygamy exists."
The immediate impact of the decision was not clear.
Legal experts disagreed whether charges could be laid against anyone in Bountiful if, as expected, the case heads for an appeal.
The province's attorney general was non-committal but also suggested an appeal could complicate matters.
Bountiful is a community of about 1,000 people in southeastern B.C., where residents follow the Fundamentalist Church of Jesus Christ of Latter-Day Saints, or the FLDS. Members of the FLDS believe polygamy will help them reach a higher level of heaven, even though the mainstream Mormon church renounced polygamy more than a century ago.
It was the failed prosecution of two of the community's leaders that prompted the constitutional case in the first place.
One of those leaders, Winston Blackmore, said the decision won't affect how Bountiful residents follow their faith.
"I certainly don't plan on dropping my faith and running away," he told The Canadian Press during an interview inside Bountiful's community hall.
"The government has tried to do everything they could in the last 20 years to ruin our lifestyle. How can the Supreme Court of Canada uphold swinging and swapping clubs? A plural relationship doesn't kill anybody. The judge, he's wrong."
George Macintosh, a lawyer appointed to oppose the law at the hearings, said he would likely launch such an appeal. He has 30 days to do so.
B.C.'s children's watchdog, Mary Ellen Turpel-Lafond, said a renewed polygamy investigation should be opened in Bountiful and charges pursued, regardless of any appeals.
"It's clear the court has said that polygamy is a criminal activity," Turpel-Lafond said in an interview. "We have a group of people who have openly professed to do this on religious grounds, the court has said it's not permitted on religious grounds, so I expect there to be open investigations."
The province's attorney general wouldn't comment on how prosecutors might proceed, particularly if an appeal is launched.
"What I think what we need to take out of today is that the court was unequivocal in upholding the concerns that we had," Shirley Bond told reporters in Victoria.
"So from my perspective, it's a strong message. We'll take some time and be practical about what steps might be taken."
In rejecting the argument that polygamy is a religious right, Bauman's answer was not short, nor simple.
It took 335 pages to explain why the current ban is constitutional, and Bauman included a number of caveats designed to fit the current law comfortably inside the confines of the charter.
He said the law is constitutional, as long as it isn't used to prosecute child brides.
It also shouldn't apply to common-law relationships involving more than two people, Bauman wrote, as long as the people involved don't formalize that relationship in a ceremony or have it sanctioned by a church or other authority.
Instead, the law is concerned only with unions that are sanctioned in some way by a civil, religious or other authority.
That definition appeared to carve out an exemption for the so-called polyamorists, who claim their relationships are consensual and egalitarian.
They wouldn't be targeted using Bauman's interpretation, although that could change if the people involved hold some sort of marriage ceremony.
Ron Skolrood, a Vancouver-based constitutional lawyer who wasn't involved in the case, said the apparent exemption for polyamorists likely wouldn't affect prosecutions in Bountiful.
"As I understand it, their claim is the right to and the belief in multiple marriages, so it might be opened to anyone to enter into consensual type relationships. But polygamy, the way its practised in Bountiful, involves that sanctioning of the union," Skolrood said in an interview.
Skolrood said it was significant that Bauman put so much emphasis on protecting the institution of monogamous marriage, which the judge said Parliament has a right to defend.
"We've seen that movement towards common-law marriage and the movement toward same-sex marriage, but those are both forms of monogamous marriage," said Skolrood.
"It seems while society has evolved in its understanding of marriage up to a point, the judge was very clear that this concept of monogamous marriage is a historical, long-standing and very important value."
Bauman was asked to answer two questions.
First, does the Criminal Code section banning polygamy violate the Charter of Rights and Freedoms? And if not, does the criminal definition of polygamy require evidence of abuse?
To the first question, Bauman said the law does indeed violate the freedom-of-religion rights of Bountiful residents, but he said the harms to women and children outweigh those rights.
"There can be no alternative to the outright prohibition," wrote Bauman.
"There is no such thing as so-called 'good polygamy.'"
For polygamous marriages involving men and women 18 and over, Bauman said the law covers all marriages that claim to be sanctioned by some higher authority, such as through a wedding ceremony or with the blessing of a church. Prosecutors don't need to show evidence of abuse or coercion, he said.
Bountiful had been investigated numerous times during the past two decades, but polygamy charges were never laid until 2009, when Blackmore and James Oler, who lead separate, divided factions within the community, were each charged with one count of practising polygamy.
Those charges were later thrown out after a judge concluded the way the province chose its prosecutors violated the men's rights.
Rather than appeal, the provincial government launched the current constitutional reference case.
The subsequent trial heard from a range of academic experts, former polygamists and current plural wives, focusing almost exclusively on Bountiful and the FLDS.
The case heard two months of testimony offering wildly diverging opinions about life in such polygamous communities and the effect of polygamy itself.
It also heard allegations that dozens of teen girls from Bountiful were sent across the border to marry much older American men, including now-FLDS leader Warren Jeffs, who is now in prison after he was convicted of sexually assaulting two teen girls.
Those allegations are now the subject of a new investigation by the RCMP, although that investigation is not looking into potential charges of polygamy.