In modern America, sex is increasingly where it should be: outside the reach of government. Anti-sodomy statutes have been tossed by the Supreme Court. Contraception is widely accessible. Anyone with a computer can gorge on pornography without fear of prosecution.

Same-sex marriage has been legalized in 18 states and the District of Columbia. Now another step has been taken to expel police and legislators from the bedrooms of consenting adults: a federal court decision striking down a key element of Utah's ban on polygamy.

Last month, District Judge Clark Waddoups ruled that the law infringes not only on constitutionally protected sexual privacy but on the free exercise of religion. Utah, he concluded, doesn't have to issue multiple marriage licenses to Kody Brown and his consorts, who appear in the reality TV show "Sister Wives." But it can't dictate their living arrangements.

The group belongs to a renegade Mormon sect that regards polygamy as sanctioned by God. Brown is legally married to one of the women and "spiritually married" to the other three. Together, at last count, they have 17 children.

If a man and a woman want to live together and call themselves partners, buddies, teammates, friends with benefits or Bonnie and Clyde, the government will leave them alone. Ditto if a guy can entice several fertile females to shack up with him and spawn a noisy horde of offspring.

But in Utah, it matters what the man calls the women living with him. If he refers to them as wives, he can go to prison. The law covers not only formal polygamous marriage but any relationships in which a married person "purports to marry another person or cohabits with another person." That was the provision ruled unconstitutional.

Laws against bigamy make sense as a safeguard against fraud. The classic case involves a cad who maintains two wives and households, each tragically unaware of the other. But no one is being fooled in the Brown home.

That makes no difference in Utah. After the TV show triggered a police investigation, state officials chose not to indict the Browns but declined to rule out future prosecution. When Kody Brown initiated a court challenge, though, the state had to acknowledge the true nature of the ban.

During the litigation, the judge addressed Assistant Utah Attorney General Jerrold Jensen: "Let's assume that a man chooses to have intimate relationships with three different women, each of whom resided in different residences, and he has children with all of them. Would that be violative of (the law)?" Answer: "I don't think that would be termed polygamy, because there's no marriage."

Suppose, said the judge, he is "married to one woman and then he has intimate sexual relationships continuing with two other women but he doesn't make any professions of commitment to these women."

Probably OK, indicated Jensen. "So it's the expression of the fact that a person is a wife that makes it illegal," surmised the judge. "Yes," Jensen replied.

This strange policy is indefensible for a host of reasons. First, it criminalizes the mere claim to be married. Second, it doesn't actually prevent men from having sex and children with multiple women. Third, it has been enforced almost exclusively against people who are motivated by religious faith.

The latter policy reflects the origin of the law: the federal demand that Utah outlaw polygamy before it could be admitted as a state. Interfering with religion was exactly the point.

In 1860, approving a law to ban plural marriage in U.S. territories, a House committee labeled Mormonism an "odious and execrable heresy." The Supreme Court later pronounced polygamy "contrary to the spirit of Christianity." They evaded the issue of religious liberty by invoking their own -- supposedly superior -- religious convictions.

The policy certainly can't be justified as a neutral attempt to uphold the interests of women and children.

"Encouraging adulterous cohabitation over religious cohabitation that resembles marriage in all but State recognition," concluded Judge Waddoups, "seems counterproductive to the goal of strengthening or protecting the institution of marriage."

There may be crimes that in practice are sometimes associated with polygamy, but the way to address those is by prosecuting the crimes -- as is the custom with non-polygamists. In any case, the judge noted, "there has been no allegation of child or spousal abuse by members of the Brown family."

Sexual freedom does not always produce results that are universally admired. But if we can tolerate Hugh Hefner's proclivities, we can tolerate Kody Brown's.