Casti connubii

The encyclical treats the goods of marriage, the errors it sees opposed to them, and the remedies. Paragraph 56 says that some had recently judged it possible to declare another doctrine, departing from the uninterrupted Christian tradition, and proclaims anew that any use of matrimony in which the act is deliberately frustrated in its natural power to generate life is an offence against the law of God and of nature. The paragraph does not name those it has in view. Paragraph 59 adds that spouses do not act against nature when they use their right in the proper manner although, for natural reasons of time or of certain defects, new life cannot result.

It is the papal teaching that Humanae vitae and the Second Vatican Council cite as their precedent. See Humanae Vitae.