I would like to add to the ltcomdata's excellent exposition of classic Catholic ethical theory with a couple of considerations.
The Church condemns contraception—that is, impeding the fecundity of an otherwise fertile sexual act—because it is harmful in various ways to the persons (particularly the married couples) who take part in it. In particular, it places a barrier to the total self-giving that spouses are called to, it dissociates the sexual act from its ultimate purpose (cooperating with God to bring new human beings into the world), and it tends to lead each spouse to view the other as an object of gratification. Moreover, it places a particularly harsh burden on women, who generally are the ones who must undergo treatment to prevent pregnancy.
Natural family planning, on the other hand, is a form of periodic abstinence, hence it is not a form of contraception at all. If done for the right reasons, it avoids all of the problems mentioned above and can even be beneficial for the unity of a marriage.
(Note that the Church does not take a firm position as to whether using contraceptives in an extramarital situation adds additional gravity to, say, sins of fornication or adultery.)
We will now look at each of these in more detail.
What does “contraception” mean?
A common misconception is that the Church condemns artificial contraception, but not “natural contraception” (which, as we will see, is a misnomer). In fact, the artificialness or naturalness of contraception has nothing to do with the Church’s condemnation of the practice.
Pope Paul VI in his landmark encyclical Humanae vitae (HV) defines contraception as
any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means (HV 14).
This echoes what Pope Pius XI wrote in his 1930 encyclical Casti connubii (CC):
any use whatsoever of matrimony exercised in such a way that the act is deliberately frustrated in its natural power to generate life is an offense against the law of God and of nature, and those who indulge in such are branded with the guilt of a grave sin (CC 56).
Interpreting some of the technical language, contraception, therefore, is defined as any act whose purpose is to render an otherwise fertile sexual act infertile. As can be seen, this can include both “artificial” methods, such as hormonal treatment or sterilization, as well as “natural” methods, such as interrupting the sexual act before it is completed.
Hence, there are three essential elements of an act of contraception:
- The couple must engage in a sexual act.
- That act must be fertile (or at least judged to be fertile), if it weren’t for the act of contraception.
- Some action must be accomplished deliberately, so as to render it infertile.
To put it in simpler language, those who contracept illicitly seek the benefit of the sexual act, without, so to speak, being willing to pay the cost, which is the possibility of procreation.
Note that the evil of contraception does not stem from its end or purpose, which is the prevention or delay of conception. Rather, the evil stems from the means used to obtain that end; i.e., frustrating the fertility of an otherwise fecund sexual act.
How contraception differs from natural family planning
This definition helps us to see how contraception is very different from natural family planning. In contrast to contraception, in NFP,
- A decision is made to refrain from engaging in the marital act, at certain moments in the woman’s cycle.
- The sexual acts that take place outside the time of ovulation are already infertile, and so they cannot be subject to contraception.
- No action is taken to render those acts infertile.
Hence, NFP fails all three conditions for contraception.
It should be noted that NFP brings with it at least two important advantages (in direct contrast to contraception): it fosters self-control, rather than indulgence; and it helps couples to communicate with each other regarding the marital act.
(I will mention in passing that natural family planning need not be a haphazard endeavor. Modern advances in gynecology make possible an accurate discovery of the woman’s fertile period, which can be used both to encourage and to prevent conception. See the Billings method and the Creighton model for more information.)
Why the Catholic Church condemns contraception
I have described what contraception is; it is now necessary to show why it is harmful.
Contraception impedes the complete self-giving of the spouses.
Married couples, as Genesis 2:24 suggests, are called to give themselves completely to one another, both spiritually and physically. The marital union involves the whole person of both spouses, but, as St. Paul points out, the physical union of the flesh is an important part of it:
For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does (1 Cor. 7:4, ESV).
The fundamental problem with contraception is that it places a barrier to the complete self-giving that spouses are called to. As Pope John Paul II in his apostolic exhortation Familiaris consortio (FC) puts it,
[Couples who have recourse to contraception] act as “arbiters” the divine plan and they “manipulate” and degrade human sexuality—and with it themselves and their married partner—by altering its value of “total” self-giving. … This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality (FC 32, quoted in part in the Catechism of the Catholic Church, No. 2370).
(Anyone familiar with the writings of Karol Wojtyła—the future John Paul II—will recognize here the echoes of his “theology of the body.” Wojtyła likened sexuality to a kind of language that necessarily implies giving the entire self to the other without holding anything back. In effect, he argues, using contraception is, in a way, a type of lie: with words, the spouses profess their total and enduring love for one another, but by holding themselves back with contraception, their actions say something different. By using contraception, couples say with their actions, “I will continue using you for my benefit, but I am unwilling to accept the consequences of that use.” See the general audience that John Paul II gave on January 12, 1983, for more on this topic.)
One sign of the degradation in self-giving is that contraception can make it more difficult for spouses to communicate regarding the marital act (and hence more difficult to communicate in general), since one of the principal reasons for such communication—namely, the discussion regarding whether to have more children—is removed.
Note that, as I mentioned, natural family planning does not have this issue; rather, it tends to promote communication and cooperation, as the spouses must plan their marital acts together. There is no question here of abusing the sexual act for personal benefit, because abstaining from it requires self-control and self-denial for the sake of the other.
Contraception dissociates the sexual act from procreation
Closely related to the first problem with contraception is the fact that it disconnects sexuality from its raison d’être, which is procreation. In the language of Casti connubii,
Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious (CC 54).
As can be seen, Pius XI subscribed to the theory by which procreation is the primary purpose of the sexual faculty; the Church does not insist on this theory, but she does hold the procreative end to be essential. In Humanae vitae’s language:
And if each of these essential qualities [of the marriage act], the unitive and the procreative, is preserved, the use of marriage [i.e., the marital act] fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which man is called (no. 12).
Since contraception effectively suppresses the procreative aspect of the sexual act, a number of important consequences follow:
- The marital act, because it is now accessible essentially without sacrifice, also becomes less attractive for the couple. (For a concise philosophical treatment on the tendency of sensual pleasure to become “saturated,” see R. Lucas Lucas, Man Incarnate Spirit: a Philosophy of Man Compendium, Circle Press 2005, pp. 218-219.)
- Breaking the link to procreation removes one of the fundamental restraints and opportunities for self-control in the the use of the marital act. That link encourages couples the think about and discuss whether they wish to accept the possibility of having another child. If they opt to delay or stop pregnancies altogether, they must make the sacrifice of abstaining for a few days a month. Breaking the link to procreation removes these positive opportunities for selfless love.
- As a consequence, contraception tends to make couples view each other increasingly as objects or as means to obtain the benefit of the marital act—which is never a positive development.
Contraception has a number of bad consequences in society
Because using contraception tends to lead to the objectification of sexuality, it has a number of consequences in society. Paul VI in Humanae vitae 17 points out three of them:
It can lead to marital infidelity and in general to sexual immorality, because it removes a natural restraint on sexual indulgence, placed there by its Creator:
Let them first consider how easily this course of action [i.e., using contraception] could open wide the way for marital infidelity and a general lowering of moral standards. Not much experience is needed to be fully aware of human weakness and to understand that human beings—and especially the young, who are so exposed to temptation—need incentives to keep the moral law, and it is an evil thing to make it easy for them to break that law.
It can lead to discrimination against and mistreatment of women:
Another effect that gives cause for alarm is that a man who grows accustomed to the use of contraceptive methods may forget the reverence due to a woman, and, disregarding her physical and emotional equilibrium, reduce her to being a mere instrument for the satisfaction of his own desires, no longer considering her as his partner whom he should surround with care and affection.
It can lead to unjust and burdensome intrusions by the state into the family:
Finally, careful consideration should be given to the danger of this power passing into the hands of those public authorities who care little for the precepts of the moral law. … It could well happen, therefore, that when people, either individually or in family or social life, experience the inherent difficulties of the divine law and are determined to avoid them, they may give into the hands of public authorities the power to intervene in the most personal and intimate responsibility of husband and wife.
(It is a confirmation of Pope Paul VI’s teaching that every one of these evils has come to pass, to varying degrees, in the decades since Humanae vitae was published.)
We could also add that the widespread use of contraception, according to Pope John Paul II in his encyclical Evangelium vitae (EV), leads to a cultural attitude—a “contraceptive mentality”—that sees children as a “burden” rather than as a gift. Such an attitude greatly facilitates the acceptance of the far greater evil of abortion:
It may be that many people use contraception with a view to excluding the subsequent temptation of abortion. But the negative values inherent in the “contraceptive mentality”—which is very different from responsible parenthood, lived in respect for the full truth of the conjugal act—are such that they in fact strengthen this temptation when an unwanted life is conceived. Indeed, the pro-abortion culture is especially strong precisely where the Church's teaching on contraception is rejected (EV 13).
Hence, to summarize the answer to the O.P.'s question:
- Contraception is defined as an action that impedes the fecundity an otherwise fertile sexual act.
- Although the fertility of sexual acts may not be frustrated, married couples may make use of the marital act even when those acts are infertile (through no fault of their own, evidently)—indeed such marital acts are to be encouraged. (See. e.g., 1 Cor. 7:5).
- Natural family planning consists fundamentally in abstaining from marital relations. It does nothing to impede the fertility of sexual acts.
- The problem with contraception has nothing to do with its end (the delay or prevention of conception), but with the fact that it tends to make one’s husband or wife a means to obtain gratification, rather than a person to be loved for his own sake.
- Contraception causes a lot of harm: to the the individuals who practice it, to married couples, and to society.
- Natural family planning, when used for the right motivation, has none of the moral problems associated with contraception, and can even be helpful for strengthening a marriage. It encourages self-control and selfless love, whereas contraception encourages indulgence and self-love.