if it is sometimes licit to tolerate a lesser evil in order to avoid a greater evil or to promote a greater good, (17) it is not licit, even for the gravest reasons, to do evil so that good may follow therefrom, (18) that is, to make into the object of a positive act of the will something which is intrinsically disorder, and hence unworthy of the human person, even when the intention is to safeguard or promote individual, family or social well-being. Consequently it is an error to think that a conjugal act which is deliberately made infecund and so is intrinsically dishonest could be made honest and right by the ensemble of a fecund conjugal life.
 

Licitness of Therapeutic Means

15. The Church, on the contrary, does not at all consider illicit the use of those therapeutic means truly necessary to cure diseases of the organism, even if an impediment to procreation, which may be foreseen, should result therefrom, provided such impediment is not, for whatever motive, directly willed. (19)
 

Licitness of Recourse to Infecund Periods

16. To this teaching of the Church on conjugal morals, the objection is made today, as we observed earlier (no. 3), that it is the prerogative of the human intellect to dominate the energies offered by irrational nature and to orientate them towards an end conformable to the good of man. Now, some may ask: in the present case, is it not reasonable in many circumstances to have recourse to artificial birth control if, thereby, we secure the harmony and peace of the family, and better conditions for the education of the children already born? To this question it is necessary to reply with clarity: the Church is the first to praise and recommend the intervention of intelligence in a function which so closely associates the rational creature with his Creator; but the affirms that this must be done with respect for the order established by God.

If, then, there are serious motives to space out births, which derive from the physical or psychological conditions of husband and wife, or from external conditions, the Church teaches that it is then licit to take into account the natural rhythms immanent in the generative functions, for the use of marriage in the infecund periods only, and in this way to regulate birth without offending the moral principles which have been recalled earlier. (20)