Married Catholics today often struggle to understand the moral difference between using contraceptives to avoid a pregnancy and using natural family planning (NFP). NFP relies on sexual abstinence during fertile periods in a woman's cycle, as assessed by various indicators like cervical mucus or changes in body temperature. To many, the Church's prohibition of contraception seems to be at odds with its acceptance of NFP because in both cases, the couple's intention is to avoid children. That intention, however, is not the problem, as long as there are, in the words of Pope Paul VI, “serious motives to space out births.” Dietrich von Hildebrand puts it this way: “The intention of avoiding conception does not imply irreverence as long as one does not actively interfere in order to cut the link between the conjugal act and a possible conception.”

That link between the conjugal act and a possible conception is a key source of meaning for our human sexuality. Sex, by it's very nature, involves the capacity and driving energy to produce offspring. Anyone in a high school biology class already understands this. We are able to recognize the purpose (or “telos”) of many different processes in the world: the telos of fire is to generate heat and to consume combustibles; the telos of an acorn is to become an oak tree; the telos of human sexuality is to draw man and woman together to procreate and raise children in the family unit. William May observes, “This is the meaning objectively rooted in the marital act itself and intelligibly discernible in it; it is not a meaning arbitrarily imposed upon or given to the act.” Seeing the telos of a process can reveal authentic goods to us which can then guide the moral choices we make.  

Any time a married couple engages in sexual activity that has been intentionally rendered infertile by contraception, they are powerfully acting against the telos of the sexual act they share. Elizabeth Anscombe notes how their act is no longer "the kind of act by which life is transmitted, but is purposely rendered infertile, and so changed to another sort of act altogether." Contraception strikes at the heart of the marital act. When a couple impedes the inherent procreative powers of that act through the use of a condom, a pill or other means, they are engaging in disruptive and contradictory behavior by seeking to perform the act on the one hand, while simultaneously blocking it on the other. 

In natural family planning, on the other hand, they are not directing any countermeasures towards the fertility of a specific conjugal act; the natural order and telos of the act is respected. As Janet Smith and Christopher Kaczor observe, “Contracepting couples make themselves infertile; NFP couples work with an infertility that is natural.”

Consider an analogy: a woman who is blind wants to talk to her husband each evening and tell him about the events of her day. He, meanwhile, wants to relax in the evenings by listening to baseball on the radio. He decides that while listening to his wife talk, he will at the same time plug in headphones and follow the game, so his attention will be divided between his wife and the game. He will occasionally says things like "yes, dear" and "uh huh" to give the impression that he is listening with full attention.

A woman on the pill similarly gives the impression that she is receiving her husband fully in the marital embrace, while, in fact, she is shutting down her own fertility in order to ward off his fruitfulness. On a deep level, she is rejecting his life-giving masculinity and speaking a false language to him with her body, much as the sports-minded husband is speaking a contradictory language with his headphones and “yes, dear” responses. If a man uses a condom with his wife, or even if both spouses agree to use contraception, they still speak a false and inauthentic language to one another right at the core of their intimacy.

Suppose that on alternating days of the week, the sports-minded husband agrees to stop listening to the radio and instead visits with his wife in a direct and focused manner. Both spouses agree to delay their gratification (he practices “sports abstinence”; she practices “verbal abstinence”), on alternating days, rather than acting against the good of their personal communication by employing countermeasures like headphones. This is similar to the case of a couple using NFP. On some days, they fully share with each other in the conjugal act; on other days, they delay sexual gratification and freely choose abstinence, so as to avoid speaking inauthentically to each other through contraceptive sex.

In sum, contraceptive intercourse always represents a radically different kind of act than intercourse during a known infertile period.

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