Forty-eight years ago this past July, our story reached a dramatic climax. But it began in the dawn of Christianity, with a document called the Teaching of the Twelve Apostles (or Didache). Written thirty to fifty years after Christ’s death, it gives the earliest evidence of a Christian condemnation of contraception. For the next 1900 years, it was the view of every Christian body—East and West, Catholic, Protestant, and Orthodox—that contraception by spouses was immoral. (Its use outside of marriage wasn’t much discussed since non-marital sex was deemed sinful anyway.) It was even denounced, vociferously, by Reformers such as Luther and Calvin. In 1930, but only then, a single Protestant denomination cracked open the door to spousal contraception—but only for serious reasons. Soon, however, that and almost every other denomination had flung it wide open.

And the Catholic Church held firm. As the sexual revolution spread and “population bomb” panic swept the West, there were rumors and fervent hopes that the Church would change. The birth control pill had just been invented, and some thought it different in kind from condoms and other barriers. Perhaps (they reasoned) it wasn’t really contraceptive. A commission established by Pope Paul VI to study the question tried to split the difference. Its 1966 report concluded that any effort to sterilize spouses’ sex acts would fall within the ancient teaching against contraception; but it urged abandoning that teaching.

Two years later, in 1968, Pope Paul VI stunned the world. His encyclical letter Humanae Vitae affirmed the historic Christian teaching against “any action which is done—either in anticipation of marital intercourse, or during it, or while its natural effects are unfolding—so as to impede procreation, whether that is intended as an end … or as a means.”

What he taught, in other words, is that it’s always immoral to act with the intent to sterilize spouses’ sexual acts, by any means and for any reason. And for good measure, he warned that a wide embrace of contraception would spell disaster for marital fidelity and public decency, for men’s respect for women and governments’ respect for the family. These words earned him the derision of Western cultural leaders in thrall to the ideology of sexual liberation, but they proved prophetic.

Paul VI also wrote—as John Paul II would reaffirm—that this principle was no mere regulation for the day-to-day life of the Catholic community, subject to change. It wasn’t like the requirement to give up meat on Fridays in Lent. It was required, they taught, by the “natural moral law.”

Why? Because a married couple’s choice to contracept goes against the human good. But there isn’t just one right account of why and how. The Church is in the business of preaching the Gospel, not running philosophy seminars. It doesn’t usually endorse particular philosophical arguments.

Nevertheless, drawing on thinkers such as Elizabeth Anscombe, Alex Pruss, and Germain Grisez, I’ll venture a few moral reasons for its teaching on contraception. I’ll show how rejecting it undermines other Christian teachings on sex ethics. And I’ll end on a more concrete note, suggesting that the use of contraception isn’t just wrong in principle; it can harm real-life marriages in tragically tangible ways.

Some dimensions of our lives are sacred, good for us in themselves. Morality requires treating these basic human goods—these core aspects of our well-being—as more than mere tools for other ends. It tells us to pursue them as we can, to honor them, and never to choose directly against them—which is simply to serve and honor human beings in these different dimensions of their lives. Thus, murder and mutilation are wrong because they involve choosing directly against the basic human goods of life and health. The inherent value of personal integrity and community makes lies and hypocrisy wrong. And so on.

In other words, the natural moral law—which Christian teaching reflects and extends—is about living well, which means loving well. It’s about serving the true good of everyone touched by our actions. It is a law of love. To act immorally, to sin, is always a failure of love, of full devotion to the human good.

Contraception Violates Marriage

The conjugal union of husband and wife—marriage—is one bedrock human good, one basic form of love. By its nature, it is deepened by the bearing and rearing of new people. But to thwart what so crowns a marriage is to choose against this good itself, against marital love. And choosing against a basic good or form of love is a sin.

How do we know that procreation deepens marriage? One clue is that the act that makes marital love is the kind of act that makes new life. If the marital act unfolds in procreation, so does the union embodied by it. What could literally em-body a couple’s one-flesh union more fully than a child, a unique and precious person who combines their bodies in one?

But to fight what naturally extends something is to fight that something. By thwarting the fruit of their lovemaking, spouses—however reluctantly or unwittingly—choose against their marital love itself. That is the first moral problem with contraception.

Now there’s a difference between thwarting a good and avoiding it. Say you have to keep a secret from a friend. You could do that by lying to him, or by avoiding him on a day when he might pry. In the first case, you thwart community by choosing what would build it up (a conversation) and then preventing it from doing so. In the second, you merely avoid the whole chance to build it up—even if the avoiding takes planning and effort. And avoiding a chance to achieve some good, on account of the costs of doing so, can’t be wrong in itself. After all, it’s inevitable. We’re forced to do that every time we face a hard choice.

Likewise, there’s nothing wrong with the goal of avoiding pregnancy for good reasons, as when having a child would make it hard to keep other commitments. Then it’s good and right to avoid sex on fertile days—even with careful planning and effort. The problem with contraception is not the goal (avoiding pregnancy), but the means. It’s not in choosing to avoid a chance to deepen your marriage, but in choosing to thwart that deepening, by having sex but sterilizing it.

A second and starker moral problem with contraception is that it makes a couple’s sex non-marital. This hasn’t been an explicit premise of the Church’s teaching but can illuminate it.

To see why, think about what sets marital love apart. What does erotic love seek? All-encompassing union with the beloved. It wants union at all levels of the person (heart, mind, and body), and in all dimensions of life (unfolding in family and home life), which therefore calls for all-encompassing commitment (lifelong and exclusive).

A union of heart, mind, and body obviously requires biological union: two becoming one flesh. Now what makes one flesh of two people? The same sort of thing that makes a single person one body, one flesh: biological coordination toward a common bodily end. Your organs make up one flesh by coordinating for their survival as a whole, the preservation of your life. Husband and wife become one flesh in biologically coordinating toward their reproduction as a couple, the making of new life.

So this link to a single biological end (reproduction) is what makes the sexual act a true bodily union at all, and one pointing to family life in particular. It’s what enables sex to give romantic love what it seeks, which is all-encompassing union with the beloved—including bodily union enriched by family life and calling for total commitment.

In a word, it is because of this natural link to that single bodily end that sex can do more than symbolize that the spouses are one at all levels, in a bond that points to family life. It can partly make them so. It seals and makes present their one-flesh union, by actually uniting them bodily. It seals their identity as a new family, by its inherent link to their reproduction.

Now marriage is not just a means to procreation. It has its own value. For a couple’s sex to be marital, they needn’t try or expect to conceive. They need only choose the right behavior, for the right reason. What’s the right reason? To embody their particular form of love: to achieve and take pleasure in their all-around union. That requires bodily union, which points us to the right behavior for a marital act: biological coordination toward a common end, reproduction. But intercourse by its nature aims at reproduction (much as the brain by its nature aids thought, and the heart circulation). So it involves some joint movement toward reproduction—achieves some bodily union—whether or not conception later occurs, or the couple expects or hopes for it. Even when a couple faces infertility, they engage in true marital acts: bodily union, chosen to embody marital love.

But when spouses try to sterilize sex, they adopt a goal at odds with its marital quality. They choose to stunt what makes this act a candidate for bodily and therefore marital union: its link to a common bodily end, to the making of new life.

And you can’t both choose and fight that one thing. A couple may want to express affection and share pleasure—and there’s nothing wrong with that! But they can’t choose a sexual act to embody truly all-encompassing union if they’re trying to stunt what makes this very act a bodily union, oriented to family life, in the first place. Even if their behavior looks the same, because they use a pill and not a barrier, they’ve crowded out the will to unite as one flesh, by willing to sterilize their act. They’re fighting the total union that romantic love seeks in this act. They might have the right behavior for a marital act, but not the right will.

And non-marital sex is wrong, no matter what makes it non-marital. It goes for shadow over substance—for the experiential layer of marital union (sexual pleasure), severed from a true embodiment of marriage. It provides climax without the behavior and will that together truly seal a marriage. This divorce of our conscious experience from our choice and action creates a divide within us. It misdirects our loves, training us to take marital delight in an act that mimics marital communion. It alienates lover and beloved from the very kind of union that they want to achieve, and therefore from each other. It dishonors marital love.

So whatever the good motives or emotional impact of a couple’s contracepted sex, it works against their marital union. (As Alex Pruss says, by fighting the union their bodies are striving for, they alienate themselves from their bodies, which prevents them from uniting in an integrated way through their bodies.) That’s why it can’t rightly embody marital affection even if it falls among acts open to life.

It may provide a sense of tenderness and intimacy, and the exhilaration of mutual exposure. But so can non-marital sex of all sorts. And like other forms of sex that don’t embody marriage—whether for lack of biological union, or of total commitment, or even of a partner—it offers a simulacrum of total union with another. A closer imitation than some other acts, but an imitation nonetheless, with the attendant alienation and dishonoring of marital love. That insight may be what led Paul VI to predict the tragic rise of divorce following the contraceptive revolution.

A Catholic slogan says it’s wrong to separate the unitive and procreative dimensions of marriage. Better to say it’s impossible. Fight what makes your sex an act of bodily union, and you won’t be choosing to come together as one flesh. Thwart the procreative, and you’ll forfeit the unitive.

Acceptance of Contraception Undercuts Christian Sexual Ethics

Christians’ approval of contraception, then, works against their other convictions on sex. If contraception is okay, then sex makes you one flesh just by making you feel closer. Any erotic behavior will do, as long as it fosters affection. But by that logic, same-sex and group sexual partnerships can achieve one-flesh union together. Yet most Christians still recognize that only a man and a woman can form a marriage.

It’s true that God forbids same-sex and multi-partner sex. But it would be arbitrary to do so while allowing contraception—and God is not arbitrary. He plays no favorites. If it were simply sexual companionship that had inherent value for us, how could he allow it for some and ban it for others? Conversely, if one-flesh union (as traditionally understood) is the true good at stake, he must forbid its imitations, no matter who the partners are. This principle makes hard demands of opposite-sex couples, and not only of those who identify as LGBT or polyamorous.

It’s also true that a husband and a wife—unlike other partnerships—can have a true marriage, open to life as a whole, even if some of their acts are not. Some Christian thinkers say that’s enough to make the contracepted acts morally good. But we all agree that an act between spouses can be wrong—e.g., if they fantasize about others while making love. Why? Because that state of will deprives this sexual act of its power to embody the couple’s marriage. That’s enough to make it non-marital, wrong in itself, even if the marriage is authentic and good. The same is true of the will to sterilize an act.

Earlier I said you could avoid a chance to embody your marriage (by avoiding sex that day) without thwarting marital love (by having but sterilizing sex). Likewise, timing your lovemaking for infertile periods will involve no non-marital sex.

Even if your timing is careful and deliberate, you do nothing at all that would deprive a single sexual act of its marital quality. (You can’t attempt to sterilize what you already know won’t lead to conception.) On fertile days, you opt for something else, like movie night. On infertile days, you achieve as much coordination toward reproduction as was ever possible in that act—i.e., only the first stage of it.

In other words, some acts move farther along the reproductive process, and others less far. But you never try to limit any act’s movement along that continuum, not even a little. So you do nothing to limit any act’s power to make you, as spouses, one flesh; none is vitiated by intent to sterilize it. Your goal is to avoid conceiving now, but your means doesn’t involve compromising your lovemaking even once.

The Ripple Effects of “Abstract” Immorality

This might all seem too abstract. How does contraception touch the day-to-day realities of marriage? Don’t marriages rise and fall on the spouses’ mutual affection and respect, on their sacrifices for each other and their children, on their emotional satisfaction? What does contraception have to do with any of this?

To begin with, immorality isn’t always about further, tangible harms. Some things are wrong in themselves, including non-marital sex. Your single act of infidelity on a business trip is wrong even if your spouse never knows and nothing else happens.

That said, contracepting often does affect spouses’ lived experience. Unsurprisingly. The pillars of a single human good—or basic form of love—support each other. We should expect eroding one to put stress on the rest. Contraception can affect other aspects of couples’ marriages and the wider culture, making a difference to people’s happiness in the most tragically concrete ways. Again, Paul VI predicted this.

To see how, start with the case of premarital sex. By severing sex from total commitment, you dishonor the good of marriage. But that core problem of moral principle can harm marriage in practice. Seeking intimacy without total commitment might dispose you to love partners conditionally, to hold a part of yourself back, maybe even to use them for your gratification. It might wear down your capacity for the sacrifices of lifelong fidelity. Research suggests that the stability of your relationships and even your sexual satisfaction could suffer in the long run. So what begins as a seemingly abstract moral problem—your willingness to sever sex from total commitment—can lead to broken hearts and homes. Not every time, but not by accident, either. These effects flow from the core, inherent wrong of premarital sex.

That’s true of non-marital sex in general. Ripple effects confirm that the feeling of intimacy in contracepted sex is, at the end of the day, a shadow of the full reality of one-flesh communion. And a few concrete harms lurk behind this shadow, too.

When spouses contracept, they may have less incentive to discern and check in with each other about the most intimate aspects of marital life (e.g., when to have kids). It might become easier over time to feel entitled to sex without consequences, and even to focus on gratifying personal desire. Children might come to seem like an optional lifestyle choice, and then an imposition, making it harder to make sacrifices for some of the unchosen demands of family life. Contraception might even dull the joy of risk and adventure and utter surrender to your beloved and your hidden future together. Not every time, again, but not by accident. For these and other reasons, divorce rates are much lower for couples who don’t contracept. Here as elsewhere, fidelity to moral truth is demanding, but it has its rewards.

If you’re skeptical of these consequences of habitual contraception, zoom out from couples to cultures. In the public mind, widespread contraception cuts the ties between sex and procreation. By promoting a sense of entitlement to sex without the risks and burdens of parenthood, it can actually increase out-of-wedlock pregnancies, along with the cultural demand for (and sense of a right to) abortion as a backup. Ideologies that divorce sex from the power to make new life also tend to reduce the value of sex to the sum of its pleasures; and of yearning for a person, to sheer thirst for an act. If sex has no meaning of its own, to be respected and not remade, there’s no objective need to reserve it for marriage. A contraceptive culture therefore tends to privatize adult desire and prioritize it over children’s needs. It swiftly comes to see moral limits beyond consent as simply baffling. These effects confirm the core moral problem with contraception.

Editor’s note: This essay first appeared August 10, 2016 on Public Discourse, the online journal of the Witherspoon Institute and is reprinted with permission. This is the first of a two-part essay. The second part may be read here. In the mid-nineteenth century illustration above, an English suffragette uses her umbrella to beat back motherhood represented by a stork.