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Marital intercourse is certainly holy, lawful and praiseworthy in itself and profitable to society, yet in certain circumstances it can prove dangerous, as when through excess the soul is made sick with venial sin, or through the violation and perversion of its primary end, killed by mortal sin; such perversion, detestable in proportion to its departure from the true order, being always mortal sin, for it is never lawful to exclude the primary end of marriage which is the procreation of children.
