There have always been forces that have pulled marriages apart. But it is the forces that push people together that are growing increasingly rare.

Social reinforcement of marriage from sources such as the workplace, the law, entertainment and the education system is fading rapidly or has collapsed completely. The church, by which I mean institutionalized Christianity in the United States, is increasingly alone in its formal privileging of marriage and family. But Christians are hardly exempt from wider mating market dynamics.

Sex has become cheap — that is, not hard to get — because it’s much less risky and consequential in the era of birth control. Perceived barriers to marriage, meanwhile, are getting higher — prompting greater marital delay and fewer marriages overall. Add to that Christians’ elevated standards for marriage and you have a recipe for wholesale retreat.

In step, many Christians’ expectations about marriage have dimmed. Whereas only 37 percent of the least religious never-married adults in the 2014 Relationships in America survey said they would prefer instead to be married, 56 percent of the most religious never-married adults said the same. But 56 is a far cry from 80 or 90 percent. Something’s going on.

Young Christians are suffering the bruising effects of participating in the same wider mating market as the rest of the country. Many Orthodox Jews and Mormons have eschewed the wider mating market, while Christians in their 20s and 30s have not. These Christians’ narratives are seldom radically different from nonreligious Americans. They want love, like nearly everyone else. They couple. Sex often follows, though sometimes after a longer period of time — a pattern that confuses them more than most, because premarital sex remains actively discouraged, but impossible to effectively prevent, in the church.

Moreover, plenty of American Christians have taken breaks from the faith, been burned, returned and then struggle to navigate new relationships in a manner distinctive from their previous mating-market experiences. (It’s not easy to raise the price of sex.) To be sure, there are those who hew to a more orthodox path — that is, dating without sex, followed by marriage in a timely fashion. They are just becoming rarer by the year.

Alternative online dating sites geared specifically toward Christians often disappoint, because their underlying template is no different. Just like secular Tinder or OkCupid, the Christian sites are guided by market-driven questions: What does he have to offer? How attractive is she? Is there a better fit out there? (Probably.) How good is “good enough”? Feigning and deception abound.

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Online dating treats humans as rank-able commodities and speeds up our ability to circulate through them. It can be navigated for noble purposes, but its baseline principles can’t really be reformed.

As marriage rates among Christians begin to decrease, additional change is afoot. Yale University sociologist Justin Farrell assessed the sexual and marital attitudes of evangelicals and found consistent age differences — younger evangelicals (below age 30) were notably more permissive on nearly all issues, especially on pornography. Critics might claim that this is nothing more than the standard age effect on sex visible from time immemorial — that older Americans have always been less permissive about sex than younger ones. However, exceptions to Farrell’s age effect are apparent among married evangelicals, meaning that under-30 evangelicals who were already married were notably less permissive. But the age at first marriage of evangelicals is climbing, in step with — about a year earlier than — -the median age of other marrying Americans (27 for women and 29 for men).

Don’t misunderstand me — American Christianity is still a friend of marriage, and vice versa. Married people make up 68 percent of all weekly church attendees between ages 24 and 35. Cohabitors barely register. Baylor University sociologist Jeremy Uecker, assessing longitudinal data from the Add Health study, concludes that cohabitation does not mix well with religious behavior. Cohabitors without children are far less likely to regularly or even sporadically attend when compared with uncoupled adults, to say nothing of married adults with children.

All this puts pressure on American pastors, operating as they are in a free religious market. How? Because it signals that they can’t count on the predictable return to organized religious life of late 20-somethings after they marry and begin having children. The return is slowed by delayed marriage. It may not occur at all, if demographer Steven Ruggles’s projection that 1 in 3 20-somethings will never marry proves true.

It’s not only in the diminished numbers of returnees that mating-market dynamics are affecting congregations. Long-standing Christian sexual ethics are making less and less sense to the unchurched — a key market for evangelicals. That’s giving church leadership fits over just how “orthodox” they can be or should be on matters of sex and sexuality. “Meeting people where they’re at” becomes challenging. Congregations are coming face to face with questions of just how central sexual ethics are to their religious life and message. The new Nashville Statement on marriage and sexuality — and emotional reactions to it — newly demonstrates just how live and poignant the tension is.

Levels of uncertainty — that is, neither agreeing nor disagreeing — about various sexual practices and attitudes are elevated among Christians. When we asked more than 15,000 Americans about sexual ethics, many who attended religious services at least once a week were on the fence. How many?

  • 23 percent are unsure about the wisdom of cohabiting before marriage
  • 14 percent are unsure about marriage being outdated
  • 21 percent don’t know what they think about no-strings-attached sex
  • 25 percent don’t know if viewing pornography is OK or not
  • 10 percent are unsure about whether extramarital sex might ever be permissible
  • 17 percent don’t know if consensual polyamorous unions are OK

One can interpret those on the fence as movable — open to being convinced. But if trends in sexual norms hold, most who once said neutrality eventually drift toward the more permissive position.

Cheap sex, it seems, has a way of deadening religious impulses. It’s able to poke holes in the “sacred canopy” over the erotic instinct, to borrow the late Peter Berger’s term. Perhaps the increasing lack of religious affiliation among young adults is partly a consequence of widening trends in nonmarital sexual behavior among young Americans, in the wake of the expansion of pornography and other tech-enhanced sexual behaviors.

Cohabitation has prompted plenty of soul searching over the purpose, definition and hallmarks of marriage. But we haven’t reflected enough on how cohabitation erodes religious belief.

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We overestimate how effectively scientific arguments secularize people. It’s not science that’s secularizing Americans — it’s sex.

Mark Regnerus is associate professor of sociology at the University of Texas at Austin and author of “Cheap Sex: The Transformation of Men, Marriage, and Monogamy.” He wrote this for The Washington Post.