[T]he 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.
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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.
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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."
Encyclical Casti Connubii (on Christian Marriage) Pope Pius XI, 1930. Paragraphs 54, 56, 61
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