My wife and I recently found ourselves comparing notes with friends who have children moving into the teenage years. We are anxious voyagers readying ourselves for stormy seas. Some of these discussions have pondered the best approach for “the talk”—on sex, dating, and marriage. And this has raised a fundamental issue for us: What is the basis for the Christian teaching on sex and marriage?

Of course, there is a strong biblical basis for the importance of marriage, from Genesis 1 to Revelation 19. But as an economist, I’m also interested in how to explain the biblical teaching on sex and the institution of marriage to millennials today. Economists like myself are infatuated with the study of institutions, but we mostly concern ourselves with rather stolid ones, like the ones that regulate capitalism. Yet in our creative displays, we engage more dynamic institutions such as dating and marriage.

When economists study institutions, we ask: What has caused such an arrangement to win out over others? Marriage is an institution that has arisen independently throughout the entire world in nearly every civilized era. But what natural basis is there, for example, to favor the institution of marriage over, say, cohabitation? What possible logic is there in criticizing something as appealing as extramarital sex? These are questions that teenagers want answers to. They will make sacrifices, but they need to know that the sacrifices make sense. Our rules and norms in local churches must be presented as rules and norms that will lead to our children’s flourishing.

I want to argue from the perspective of social science that the Christian teaching on sex and marriage is much more than a dated rule that ruins the fun of teenagers and adults. Rather, behind marriage lies a social justice issue related to biological asymmetries between men and women. Keeping sex within the context of a lifetime commitment creates the basis for a healthy relationship between the genders.

A Form of Stealing

From a biological perspective, both males and females are equally concerned with successful gene reproduction. But there are important asymmetries in how males and females meet this goal. Ignorance of these asymmetries is where much of today’s bad advice on sex and marriage originates.

Mainstream Western culture generally advocates a kind of psychological androgyny. The psychological differences between men and women, it is argued, are not innate but culturally created. Most Christians reject this view, partly because we recognize beauty in the differences in our psychologies that stem in part from our distinctive biologies. Biologically, females typically bear only one offspring at a time. As a result, a woman’s interest lies in the quality of that offspring and the health, protection, and resources required to help an infant flourish. It is possible for males, however, to reproduce their genes by producing many offspring at a time with many females. To put it bluntly, this gives males a different set of incentives in sex. The biological interest of women is in quality of sexual relationship; in men it is in quantity of sexual relationship.

Thus, at its most base level, a sexual relationship between a man and a woman involves an exchange of sex for commitment, commitment on behalf of the man to the welfare of the woman and any resulting children. Seen in this light, the commitment of a man to a woman with whom he has a sexual relationship is not prudery; it is social justice. From a biological standpoint, sex devoid of genuine male-to-female commitment is a form of stealing. And a widespread social acceptance of sex without commitment represents an injustice against women and their deepest biological interests. The tragic irony is that the “sexual liberation” espoused by some secular feminists couldn’t play more perfectly into the short-term, selfish interests of men.