Leaders of the United Church of Christ are incensed that two TV networks, CBS and NBC, are refusing to air a commercial celebrating the denomination's "all-inclusive welcome," not least toward gays and lesbians. Network executives call the ad "too controversial," while church leaders cry censorship. Both sides are missing an opportunity to elevate the debate about gay marriage.

The 30-second ad shows a beefy bouncer working a rope line outside a church. He's keeping various people out: Latinos, African-Americans and gay couples. Words flash across the screen: "Jesus didn't turn people away. Neither do we." The scene shifts to the inside of a UCC church, with an obviously diverse and happy congregation. Two women embrace in the final scene.

UCC officials are explicit about the ad's discrimination theme. "In the 1960s, the issue was the mixing of races. Today the issue appears to be sexual orientation," says Ron Buford, coordinator for the UCC campaign. "In both cases, it's about exclusion." In other words, according to UCC logic, churches that uphold traditional marriage are on par with the racists of the Jim Crow South. Call it faith-based bigotry.

That kind of slur was apparently too much for NBC, whose spokeswoman said that the network objected not to the portrayal of same-sex couples in church but to the insinuation that other faith traditions routinely discriminate. Both CBS and NBC also have policies banning "advocacy" ads and cite the current debate over the federal marriage amendment. The ad has been accepted by other broadcast and cable networks, however, including ABC Family, BET, Fox, TNT and Nick@Nite.

Nevertheless, the UCC smells censorship -- and worse. "By refusing to air the United Church of Christ's paid commercial, CBS and NBC are stifling religious expression," says UCC spokeswoman Gloria Tristani. Such decisions, she says, put freedom of speech "in jeopardy." That's overheated. Media outlets have a First Amendment right to reject messages they find too controversial, misleading or inflammatory.

Still, the two networks might want to rethink their decision and air the ads -- as long as they offer equal time to competing views. The problem with media coverage of the debate over homosexuality is that it's so intellectually deficient: The most extreme voices get most of the air time. The "traditional" view, when it's heard, usually amounts to a red-faced minister quoting from Leviticus to explain why "God hates gays." Paid TV spots would give churches and other religious groups a chance to craft their messages carefully, free of the caricatures that drive network coverage of religion.

Giving advertising time to religious viewpoints would also help counter the impulse to stigmatize traditional religious ideals. It's a growing problem: At a gathering of pro-gay activists in Geneva earlier this year, I heard a United Nations official compare the agenda of traditional marriage groups to the Nazi campaign against homosexuals. Everyone in the room nodded in agreement.

Indeed, the UCC ad symbolizes the mischief created by the partnership between liberal religion and "progressive" causes. Sometimes that alliance has been a constructive force, as in the civil-rights movement. But too often it has fueled ideas that undermine the family, religious liberty and civil society -- from the eugenics movement of the 1920s to the Marxist "liberation" groups of the 1960s and '70s. In their attempt to make the gospel "relevant" to contemporary culture, progressive churches have appeared irrelevant to more and more Americans.

Hence the UCC ad, part of a $1.7 million campaign to boost the visibility of the church. Over the past 15 years, membership has declined 23% to barely 1.3 million churchgoers. Ted Pulton of Gotham Inc., an advertising agency advising the UCC campaign, describes the church's name recognition as "negligible at best."

The ad campaign could increase brand recognition, but will that translate into a larger market share? Will the denomination's embrace of gay marriage embolden the faithful and expand the flock? A warning from William Inge, dean of St. Paul's Cathedral, comes to mind: "He who marries the spirit of the age will soon find himself a widower."

Mr. Loconte is a fellow at the Heritage Foundation and editor of "The End of Illusions: Religious Leaders Confront Hitler's Gathering Storm" (Rowman & Littlefield).