Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called "indications," may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life. From this it follows that the observance of the natural sterile periods may be lawful, from the moral viewpoint: and it is lawful in the conditions mentioned. If, however, according to a reasonable and equitable judgment, there are no such grave reasons either personal or deriving from exterior circumstances, the will to avoid the fecundity of their union, while continuing to satisfy to tile full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles.
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The primary end of marriage
Now, the truth is that matrimony, as an institution of nature, in virtue of the Creator's will, has not as a primary and intimate end the personal perfection of the married couple but the procreation and upbringing of a new life. The other ends, inasmuch as they are intended by nature, are not equally primary, much less superior to the primary end, but are essentially subordinated to it. This is true of every marriage, even if no offspring result, just as of every eye it can be said that it is destined and formed to see, even if, in abnormal cases arising from special internal or external conditions, it will never be possible to achieve visual perception.