It does not follow from Catholic principles that conjugal intercourse is forbidden whenever conception is naturally impossible, as when a woman is already pregnant or advanced in years. Nature itself includes such conditions in its plan, and so in these circumstances a married pair do nothing against nature, nothing immoral, if they make use of their marital rights, provided they have the power of complete coition. They are not positively frustrating the chief purpose of the sexual act, they are not opposing the designs of nature and of nature's Author.
It is important to note that the argument which is being urged against contraception is not based on any such principle as 'It is always sinful to oppose or to check any force of nature.' The misunderstanding of this point has occasioned innumerable objections of the species known as reductio ad absurdum against the Church's denunciation of contraceptive practices. For example: 'The Catholic teaching on birth control would lead to the conclusion that a person commits a sinful act whenever he cuts his hair or trims his nails, since in performing these actions one frustrates nature.'
The flaw in this manner of reasoning is the failure to distinguish between the restricting of a natural power and the preventing of the purpose of a natural power. The former by no means necessarily includes the latter. It is within the designs of nature itself that there should be opposition and conflict among the multitudinous forces and agents that operate in the universe, that one creature should restrain and control the tendencies and activities of another and utilize them to its own advantage. The animal violently interrupts the vital functioning of the plant by using it as food, and man does the same to the animal; but there is no frustration of any divinely ordained purpose in this process. On the contrary, there is the fulfillment of the Creator's design that the lower in the scale of perfection should contribute to the sustenance of the higher. Similarly -- to answer the specific objection -- when a person cuts his hair or trims his nails he does indeed curtail the growth of these bodily appendages, but their chief purpose, the utility of the individual himself, is promoted rather than frustrated. Certainly nature does not call for an unchecked augmentation of hair and nails; they must be clipped if they would be beneficial to the whole person, to whom they are subservient as the lesser good to the greater. But it is an utterly different case with contraception, which prevents the very primary purpose of sexual activity and inverts the due order of things by making the social benefit of conjugal intercourse subservient to the benefit of the individuals concerned.
This can be illustrated by a development of the parallelism which exists between the faculty of nutrition and that of sex. The primary purpose of the former is to preserve the life of the individual; the primary purpose of the latter is to preserve the life of the human race. To attract human beings to the due use of these faculties, the Creator has annexed to the functioning of each a feeling of pleasure. Sexual gratification is particularly vehement, and in this is manifest the sagacity of divine providence, inducing men and women to undertake the arduous duties of parenthood for the benefit of the human race. But, to continue the analogy, it is possible for a person to enjoy the pleasure accompanying the use of either of these faculties, and at the same time to distort his action in such wise that its chief purpose is rendered unattainable. This is what takes place relative to the sexual faculty when contraception is employed. And the analogous case in the use of the nutritive faculty is the revolting practice of some ancient Roman gourmands, who ate to satiety and then induced regurgitation. In each case the sensual gratification intended by the Creator as an incentive to the use of the respective faculty is sought and enjoyed, while the divinely established main purpose is deliberately and positively obstructed. Is it not strange that many persons who shudder at the very thought of the disgusting custom of the ancient voluptuaries do not hesitate to defend and to practise the equally perverse operation of contraception?