Purity, or chastity, is the virtue (stable or habitual disposition) which firmly inclines a person towards the right use (or non-use) of the sexual faculty. Chastity in marriage means using the human capacity for sex according to its authentic nature and purpose; and, as Pope Paul VI enunciated in his 1968 Encyclical Letter Humanae Vitae, the ends of marriage, or more specifically, of the “marital act”, are “the union of the spouses and, inseparably therefrom, procreation” (HV, No.12). Thus, a rule-of-thumb: sex is good, but it must be within the context of an exclusive and indissoluble marriage and it must be open to life.
Sex is ordained by nature for procreation— “reproductive system” is the term for the collection of body parts involved—and it is an abuse, a moral disorder, to use it in denial of its procreative end. Thus, contraceptive sex is immoral, just as it is immoral to eat for the sake only of satisfying the appetite, in denial of the end proper to the “digestive system”, which is the nourishment and preservation of the individual's life. The pleasures involved in eating, and in sex, are nature's ways of helping man to achieve necessary ends he might otherwise neglect to his injury. Parenthetically, while contraceptive sex is immoral, natural family planning methods are morally licit precisely because they consist in the non-use of the sexual faculty during the fertile periods—there is no abuse.
Of course, sex is different from eating: “For, unlike food, which is necessary for the individual, procreation is necessary only for the species, and individuals can dispense with it.” (The Way, No. 28) That individuals can dispense with sex is affirmed by those who have lived holy celibacy.
Eating is directly ordained to the good of the individual, while sex is oriented to the good of the family and society (preservation of the species). This is why the immorality of contraceptive sex is less obvious (to the individual) than that of gluttony (the unbridled appetite for food and drink, where induced vomiting, to allow one to continue eating or drinking, is the moral equivalent of contraception). The undesirable consequences of the abuse of the sexual faculty (on family and society) may not immediately affect the individual actor, unlike those arising from over-eating or drunkenness. Also, the pleasure involved in sex is more intense than in eating, as nature's way of compensating the individual for serving the social purpose. Most people would still eat, knowing they need to in order to live, even with only the gentle prodding of the appetite; but very few would think of marriage and raising a family without the strong urge accompanying the marital act. Even so, the analogy stands: gluttony leads to the death of the individual; sex in denial of its procreative purpose leads to the death of the family and society.
It can be argued that the purpose of the human sexual faculty is served already in the loving union of the partners; and that there is no need to advert to procreation. But then, to deny the procreative end of sex would remove the rational basis for the very existence of the institution of marriage. Indeed, an indissoluble marriage is necessary precisely because sex is intended by nature for procreation, including the upbringing and education of the offspring—so fragile in the developing stages—which requires a lasting partnership of the father and mother, i.e., lasting independently of the changeable preferences and circumstances of the parties. If sex is not for procreation, the institution of marriage would be meaningless.
It is precisely because of the obvious procreative purpose of sex that even primitive cultures have some kind of marriage institution. The common good (perpetuation of the tribe) requires a social “mechanism” to ensure the welfare of the offspring. Thus, to isolate the conjugal act from its procreative purpose, so as to justify contraceptive sex, would also justify divorce (no need for permanence in the partnership of the spouses) and homosexuality (no need for procreation), and so would open the floodgates for social disaster—broken homes, juvenile delinquency, the AIDS epidemic, etc., not to mention the problems of shrinking or aging populations in those nations that have early on adopted birth-control policies, confusing issues of social-justice, economics, etc., with supposed over-population.
All human life is good. Pregnancy is not a disease. Responsible parenthood does not consist in having few children but in the efforts of parents to discharge their vocation to the procreation and education of offspring. And peoples and nations do not become richer or happier because their members are few.
St. Josemaria writes: “We all know from experience that we can be chaste, living vigilantly, frequenting the sacraments and stamping out the first sparks of passion before the fire gets started. And it is precisely among the chaste where the most clean-cut men from every point of view are found. And among the impure abound the timid, the selfish, the hypocritical and the cruel—all characters of little manliness.” (The Way, No. 124) And chastity in marriage means respecting its ends: “No Christian married couple can want to block the well-springs of life. For their love is based on the Love of Christ, which entails dedication and sacrifice. Moreover, as Tobias reminded Sara, a husband and wife know that ‘we are children of saints, and we cannot come together in the way of the Gentiles, who do not know God’.” (Furrow, No. 846)