In 2019, my then-fiancée and I met with our priest during the premarital counselling process. One of his questions was whether we wanted children. We said we did. That was good, he replied. If we didn’t, he would refuse to marry us. Although he would make exceptions for marriages beset by genetic, medical, or mental health issues, he flatly declared that DINK (Dual Income No Kids) couples were going against God’s design for marriage.

I was shocked. I would have expected such a position from a Catholic priest, but our priest is Anglican. Besides, some people just don’t want kids, I reasoned, and that’s their decision to make.

Or is it?

To answer that question, we must first ask two others: “What is marriage for?” and “What is sex for?”

Or do those questions even make any sense at all? From the dominant perspective of our culture, they don’t, because today’s culture has largely accepted Jean-Paul Sartre’s assertion that, for human beings, “existence precedes essence.” Sartre explains that whereas something like a knife is made with a particular purpose in mind—i.e. cutting things—humans have no such purpose. Instead, the argument goes, we are the product of blind evolutionary forces and must therefore determine our own destiny.

From this perspective, the answer to the questions “What is marriage for?” and “What is sex for?” is, “Whatever you decide!” In most modern sitcoms, one character gets cold feet about getting married and another character talks him down off the ledge by explaining that, yes, marriage is an objectively meaningless and probably outdated institution, but it’s possible to make marriage meaningful by committing yourself to it. “Maybe even the belief in it is more important than the thing itself,” Diane Nguyen explains in an episode of BoJack Horseman.

This is likely why so many couples now write their own wedding vows. Instead of using traditional vows to signal that marriage is something that has a set definition and is bigger than either of them, the couple shifts all of the focus to the uniqueness of their relationship, thereby implying that marriage is whatever they say it is.

Sex gets the same treatment. Sure, sex might serve a particular biological or evolutionary purpose, but now we have the technology to totally separate sex from procreation, so we do. It’s not as if random evolutionary forces can make any sort of claim on us, after all. Thus sex, like marriage, comes to mean whatever we want it to mean.

This attitude is displayed in one episode of the sitcom How I Met Your Mother, when the characters gleefully list 50 different reasons to have sex, each apparently just as valid as any other. Love, lust, boredom, envy, curiosity—all are perfectly legitimate motives for sleeping with someone. You’re free to do whatever feels good, as long as you don’t hurt anyone else.

But for anyone who believes that there is such a thing as human nature, that there is a goal at which we ought to aim, and that it is possible to “miss the mark” (the literal definition of “sin”), the questions “What is marriage for?” and “What is sex for?” make perfect sense. People who believe these things strive to live in accordance with truth and to abstain from telling—or, worse, living—lies. The idea that we can and ought to bend sex to whatever purpose we choose is a lie, and a particularly nefarious one since it misrepresents one of the most important aspects of human life.

Fundamentally, sex does two things. First, it makes babies. Second, it produces physical pleasure and feelings of intimacy, which tend to forge a bond between partners. Today, both are considered undesirable and are referred to as “getting pregnant” and “catching feelings,” though the Catholic Church prefers to describe them as the “procreative” and “unitive” aspects of sex. To engage in sex while fully disavowing either is to tell a lie about what sex is, to refuse to be what we are and ought to be, to miss the mark, to sin.

In practical terms, don’t have sex with people with whom you aren’t prepared to have children and don’t have sex with people with whom you aren’t prepared to form a lasting emotional bond. You might be able to avoid both outcomes, maybe even for your whole life, but to succeed at living a lie is no triumph at all.

Marriage is nothing more than the acknowledgement that sex is both unitive and procreative. If sex binds people together and makes babies, then it makes sense that we should bind people together to make babies so that they stay together in order to provide those babies with stable homes in which to grow up.

The evangelical purity movement, which encourages teens to save sex for marriage, often gives kids the impression that once they’re married, all the rules—aside from “Thou shalt not commit adultery”—go out the window. But it’s possible to be faithfully married and still live a lie about what sex is. One way to do that is to deny the unitive aspect of sex by using your spouse for physical pleasure without considering whether he or she is enjoying it. Another way, which I learned about from my priest, is to render your marriage artificially barren, thereby denying that sex is procreative.

There are complicated debates to be had about which methods couples can morally use to space apart pregnancies or limit the total number of children they have, but to anyone who rejects the Sartrean assertion that existence precedes essence, it should be clear that to marry without wanting children is to enter into a distorted form of marriage. Because although “unitive” and “procreative” are useful terms, they’re really just two ways of stating the same truth. The purpose of sex and of marriage is to build families.