17. Since, however, We have spoken fully elsewhere on the Christian education of youth, let Us sum it all up by quoting once more the words of St. Augustine: "As regards the offspring it is provided that they should be begotten lovingly and educated religiously," - and this is also expressed succinctly in the Code of Canon Law - "The primary end of marriage is the procreation
53. And now, Venerable Brethren, we shall explain in detail the evils opposed to each of the benefits of matrimony. First consideration is due to the offspring, which many have the boldness to call the disagreeable burden of matrimony and which they say is to be carefully avoided by married people not through virtuous continence (which Christian law permits in matrimony when both parties consent) but by frustrating the marriage act. Some justify this criminal abuse on the ground that they are weary of children and wish to gratify their desires without their consequent burden. Others say that they cannot on the one hand remain continent nor on the other can they have children because of the difficulties whether on the part of the mother or on the part of family circumstances.
54. But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. 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.
55. Small wonder, therefore, if Holy Writ bears witness that the Divine Majesty regards with greatest detestation this horrible crime and at times has punished it with death. As St. Augustine notes, "Intercourse even with one's legitimate wife is unlawful and wicked where the conception of the offspring is prevented. Onan, the son of Juda, did this and the Lord killed him for it."
56. Since, therefore, openly departing from the uninterrupted Christian tradition some recently have judged it possible solemnly to declare another doctrine regarding this question, the Catholic Church, to whom God has entrusted the defense of the integrity and purity of morals, standing erect in the midst of the moral ruin which surrounds her, in order that she may preserve the chastity of the nuptial union from being defiled by this foul stain, raises her voice in token of her divine ambassadorship and through Our mouth proclaims anew: 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.
57. We admonish, therefore, priests who hear confessions and others who have the care of souls, in virtue of Our supreme authority and in Our solicitude for the salvation of souls, not to allow the faithful entrusted to them to err regarding this most grave law of God; much more, that they keep themselves immune from such false opinions, in no way conniving in them. If any confessor or pastor of souls, which may God forbid, lead the faithful entrusted to him into these errors or should at least confirm them by approval or by guilty silence, let him be mindful of the fact that he must render a strict account to God, the Supreme Judge, for the betrayal of his sacred trust, and let him take to himself the words of Christ: "They are blind and leaders of the blind: and if the blind lead the blind, both fall into the pit.
58. As regards the evil use of matrimony, to pass over the arguments which are shameful, not infrequently others that are false and exaggerated are put forward. Holy Mother Church very well understands and clearly appreciates all that is said regarding the health of the mother and the danger to her life. And who would not grieve to think of these things? Who is not filled with the greatest admiration when he sees a mother risking her life with heroic fortitude, that she may preserve the life of the offspring which she has conceived? God alone, all bountiful and all merciful as He is, can reward her for the fulfillment of the office allotted to her by nature, and will assuredly repay her in a measure full to overflowing.
59. Holy Church knows well that not infrequently one of the parties is sinned against rather than sinning, when for a grave cause he or she reluctantly allows the perversion of the right order. In such a case, there is no sin, provided that, mindful of the law of charity, he or she does not neglect to seek to dissuade and to deter the partner from sin. Nor are those considered as acting against nature who in the married state use their right in the proper manner although on account of natural reasons either of time or of certain defects, new life cannot be brought forth. For in matrimony as well as in the use of the matrimonial rights there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider so long as they are subordinated to the primary end and so long as the intrinsic nature of the act is preserved.
60. We are deeply touched by the sufferings of those parents who, in extreme want, experience great difficulty in rearing their children.
61. However, they should take care lest the calamitous state of their external affairs should be the occasion for a much more calamitous error. No difficulty can arise that justifies the putting aside of the law of God which forbids all acts intrinsically evil. There is no possible circumstance in which husband and wife cannot, strengthened by the grace of God, fulfill faithfully their duties and preserve in wedlock their chastity unspotted. This truth of Christian Faith is expressed by the teaching of the Council of Trent. "Let no one be so rash as to assert that which the Fathers of the Council have placed under anathema, namely, that there are precepts of God impossible for the just to observe. God does not ask the impossible, but by His commands, instructs you to do what you are able, to pray for what you are not able that He may help you."
62. This same doctrine was again solemnly repeated and confirmed by the Church in the condemnation of the Jansenist heresy which dared to utter this blasphemy against the goodness of God: "Some precepts of God are, when one considers the powers which man possesses, impossible of fulfillment even to the just who wish to keep the law and strive to do so; grace is lacking whereby these laws could be fulfilled."