By now, we’ve all heard the refrain that U.S. churches need liberalize their teachings on sexuality and homosexuality or rapidly decline. The logic behind the argument is simple: more and more Americans are embracing homosexuality and same-sex marriage, including growing numbers of religious Millennials. So long as churches remain the face of opposition to gay marriage, those churches will shrink into irrelevancy when gay marriage (inevitably, we are told) becomes a settled political issue. These arguments often see church acceptance of homosexuality as a carrot as well as a stick. It isn’t so much that denouncing homosexuality will drive people...