In Defense of the Family: Part Nine - Humanae Vitae Paragraph 14

Unlawful ways of regulating birth

"Therefore we base our words on the first principles of a human and Christian doctrine of marriage when we are obliged once more to declare that the direct interruption of the generative process already begun and, above all, direct abortion, even for therapeutic reasons, are to be absolutely excluded as lawful means of controlling the birth of a child.

Equally to be condemned, as the Magisterium of the Church has affirmed on various occasions, is direct sterilization, whether of the man or of the woman, whether permanent or temporary

Similarly excluded is 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.

Neither is it valid to argue, as a justification for sexual intercourse which is deliberatively contraceptive, that a lesser evil is to be preferred to a greater one, or that such intercourse would merge with normal relations of past and future to form a single entity, and so be qualified by exactly the same moral goodness as these. Though it is true that sometimes it is lawful to tolerate a lesser moral evil  in order to avoid a greater or in order to promote a greater good, it is never lawful , even for the gravest reasons, to do evil that good may come of it. - in other words, to intend positively something which intrinsically contradicts the moral order, and which must therefore be judged unworthy of man, even though the intention is to protect or promote the welfare of an individual, of a family or of society in general. Consequently it is a serious error to think that a whole married life of otherwise normal relations can justify sexual intercourse which is deliberatively contraceptive and so intrinsically wrong."