(RNS) — Gay marriage is now the law of the land and, increasingly, a line in the sand conservative churches say they will not cross.

The latest is the Church of Jesus Christ of Latter-day Saints. It issued new guidelines last week, saying Mormons in same-sex relationships will face possible excommunication and their children will not be permitted to join the church until they are 18 — and then only if they reject their parents’ relationship.

The new policies make the LDS church, with about 15 million members globally, the largest Christian denomination to enact such a formal — and some say draconian — policy. Children of same-sex couples are effectively denied baptism and other ordinances of the church.

Related: Marriage Under ‘Attack’

“I am sure everyone feels regret about this,” said D. Michael Quinn, a scholar who was excommunicated by the LDS church in 1993, but still considers himself a Mormon. “The leaders who have instituted this have done so without any relish. They have done so with regret. All Mormons down the line will look at it as regrettable.”

Mormons are not the only religious group that strongly rejects gay marriage. The Roman Catholic Church does not condone same-sex marriage, and its priests may not perform them. The United Methodist Church, the largest mainline Protestant denomination, does not embrace same-sex marriage, and neither do many evangelical groups.

Mormons believe they are children of heavenly parents and that only by being married to a member of the opposite sex can they become like their Heavenly Father and Heavenly Mother and be eternally exalted.

But none of those groups has gone so far as to have a formalized policy targeting children of gay couples. While local Catholic officials have fired teachers and other church employees in same-sex marriages, Pope Francis has called for recognizing the “gifts” of LGBT Catholics. And while a majority of evangelicals reject gay marriage, a growing minority supports LGBT rights.

There were indications the LDS church was on a similar path. In 2012, it established the website Mormons and Gays and officially rejected the idea that same-sex attraction is a choice. Several church officials made statements interpreted as supportive of LGBT Mormons, and last month, Dallin Oaks, a high-ranking LDS church official, said Kentucky’s Rowan County Clerk Kim Davis was wrong to use her Christian faith as a reason to refuse to issue marriage licenses to gay couples.

Related: Rabbis Weigh in on Marriage

The new policy has brought a feeling of whiplash, said Randall Thacker, president of Affirmation, a support and advocacy group for LGBT Mormons and their families. “Their credibility is lost,” he said of church officials. “There is a feeling in the Mormon community that they can’t trust their leaders.”

Who do you think would win the Presidency?

By completing the poll, you agree to receive emails from LifeZette, occasional offers from our partners and that you've read and agree to our privacy policy and legal statement.

But in a video statement defending the new guidelines, LDS Elder D. Todd Christofferson said the Supreme Court’s June 26 ruling on gay marriage made them necessary.

“There was the need for a distinction to be made between what may be legal and what may be the law of the church and the law of the Lord,” said Christofferson, a member of the Quorum of the Twelve Apostles, the church’s governing body. “It’s a matter of being clear. It’s a matter of understanding right and wrong. It’s a matter of a firm policy that doesn’t allow for question and doubt.”

The new policy has some precedence within the church.

In the late 1970s, as the church expanded across sub-Saharan Africa, where polygamy is legal, it required that men who wished to join the church dissolve their extra marriages. Quinn, who has studied Mormonism and polygamy, said whole families were sundered, legal wives were shamed and legitimate children were abandoned.

Another reason for the same-sex guidelines is the view of marriage in the LDS theology. Mormons believe they are children of heavenly parents and that only by being married to a member of the opposite sex can they become like their Heavenly Father and Heavenly Mother and be eternally exalted. Mormon scripture states, “If a man marry a wife by my word, which is my law … then shall they be gods.”

This article originally appeared in Religion News Service.