LOUISVILLE, Ky. — Christians should not attend a same-sex wedding — even of their own child — because it signals "moral approval" of the union, the president of the Southern Baptist Theological Seminary said in a new book.

Boycotting weddings of gay friends and loved ones will be excruciatingly difficult, the Rev. Albert Mohler wrote in "We Cannot Be Silent," which goes on sale Oct. 27.

“At some point, attendance will involve congratulating the couple for their union," he wrote. "If you can’t congratulate the couple, how can you attend?”

Even if scientists prove people are born gay, the “sinfulness of homosexuality” would not be eliminated because human sin taints the world, Mohler said in addressing other topics of sexual identity. He contended that transgender people who are "saved" should consult with their pastors about whether to have surgery to return to their original gender.

Mohler, a prolific blogger, speaker and podcaster, is no stranger to controversy. In the past he has said that delaying marriage and limiting family size are both sins, that the Roman Catholic Church teaches a “false gospel,” that the pope holds an “unbiblical office,” and that Christians who practice yoga “either deny the reality of what yoga represents or fail to see the contradictions between their Christian commitments and their embrace” of it.

He wrote his 213-page book, subtitled “Speaking truth to a culture redefining sex, marriage & the very meaning of right and wrong,” for what he called “intelligent evangelical readers,” pastors, other church leaders and the public, he said.

Gay-rights activists and some clergy denounced the book, to be published by HarperCollins Christian Publishing, saying it will further divide gays and their families.

“Dr. Mohler's self-righteous intractability on this issue — even banning followers from simply attending the weddings of their LGBT loved ones — can cause nothing but strife, heartache and hardship,” said director Chris Hartman of the Fairness Campaign, a Louisville-based non-profit advocacy organization that works to prevent discrimination based on sexual orientation and gender identity.

Southern Baptists are the largest Protestant denomination in the USA, numbering a little less than 15.5 million members, down from a peak of 16.3 million in 2003.

READ: What to make of Southern Baptists’ declining numbers (COMMENTARY)

While no official tally of the number of lesbian, gay, bisexual and transgender Americans exists, a Gallup poll released in May estimates the national average at 3.8 percent of the U.S. population, which works out to about 12.2 million people.

The Rev. Joseph Phelps, pastor of the independent Highland Baptist Church in Louisville, praised Mohler’s intellect but called his words harsh and offensive and said they will cause damage and division in families and society.

Whether to attend or avoid same-sex marriages has been a hot topic in conservative Christian circles. While many evangelical leaders have sided with Mohler, others have urged Christians to go.

“I have to extrapolate that Jesus would be all for attending a same-sex marriage ceremony,” based on Jesus' example of dining with prostitutes and drunkards, Stephen Arterburn, a best-selling Christian author who is president of New Life Ministries and host of the radio and television show "New Life Live," said in a recent column.

Attending a same-sex wedding doesn’t imply approval but instead personal support and love for the marrying couple, according to the Chicago-based Marin Foundation, which works to build bridges between the gay community and conservatives.

READ: Like it or not, most expect gay marriage will sweep the US

Some Catholic bishops have urged parishioners not to attend gay weddings while others have not addressed the question.

While Jesus regularly ate with sinners, Mohler said that “his constant call was to repentance” and in no case did he endorse sin.

Mohler said he never has attended a same-sex wedding and wouldn't even if one of his children or grandchildren were marrying. In an interview, he said he addressed the question in the book because students and grandparents had asked about it even before the June 26 Supreme Court decision legalizing gay marriage.

"I don’t want to underestimate the difficulty of these questions, but I don’t think a faithful Christian can recognize or celebrate … what we don’t think is a marriage,” he said.

Most of Mohler's new book was written before the Supreme Court ruling. In an afterword, Mohler, who has led Southern Baptist seminary for 22 years, said the decision vilifies those with a traditional view of marriage and places every religious institution in jeopardy.

READ: No ‘therapy,’ but Jesus can change LGBT lives, say evangelical leaders

Yet Mohler doesn't propose that conservative Christians isolate themselves from gay community.

In one chapter, Mohler asks and answers what he calls hard questions about gay sexuality, such as whether it is OK for Christian parents to let their kids play at the home of children with gay parents. He says it is, as long as the parents instruct their child first on “scriptural authority and sexuality.”

“We should make every effort to develop real and authentic friendships with our LGBT neighbors,” he wrote.

Timothy Love, one of Kentucky's plaintiffs who won the Supreme Court ruling legalizing gay marriage nationwide, said Mohler's advice on boycotting weddings is ironic.

He and Larry Ysunza, who were married Saturday, had a Christian wedding and both have been Christians all of their lives, Love said.

"We are closer to God now more than ever because he has shown us a miracle in modern times, right before our very eyes, and allowed us to have a role in it,” he said. “We look back at the long timeline of our relationship (35 years) and truly know that there has been a hand over us protecting us through this difficult journey. There are none so blind as those who will not see."

(Wolfson reports for The Louisville Courier-Journal.)

LM END WOLFSON