It’s time to get our hands dirty by digging into the writings of recent popes to find out what they had to say about contraceptive issues. Let’s start with Pius XI’s 1930 Casti Connubii, which was written in response to the Anglican Communion’s decision that year to permit artificial contraception within marriage (general acceptance came later).
Skipping past the introductory stuff,
“24. This mutual molding of husband and wife, this determined effort to perfect each other, can in a very real sense, as the Roman Catechism teaches, be said to be the chief reason and purpose of matrimony, provided matrimony be looked at not in the restricted sense as instituted for the proper conception and education of the child, but more widely as the blending of life as a whole and the mutual interchange and sharing thereof.”
Summary: the purpose of marriage is more than the bearing and raising of children.
“25. By this same love it is necessary that all the other rights and duties of the marriage state be regulated as the words of the Apostle: ‘Let the husband render the debt to the wife, and the wife also in like manner to the husband,'[1 Corinthians 7:3] express not only a law of justice but of charity.”
In other words, don’t unjustly deprive your spouse of sex. It should be noted that St. Paul mentions only spending time in prayer as a reason for deliberately avoiding sex, saying, “Do not refuse one another except perhaps by agreement for a season, that you may devote yourselves to prayer; but then come together again, lest Satan tempt you through lack of self-control.” (1 Corinthians 7:5).
“53. And now, Venerable Brethren, we shall explain in detail the evils opposed to each of the benefits of matrimony. First consideration is due to the offspring, which many have the boldness to call the disagreeable burden of matrimony and which they say is to be carefully avoided by married people not through virtuous continence (which Christian law permits in matrimony when both parties consent) but by frustrating the marriage act. Some justify this criminal abuse on the ground that they are weary of children and wish to gratify their desires without their consequent burden. Others say that they cannot on the one hand remain continent nor on the other can they have children because of the difficulties whether on the part of the mother or on the part of family circumstances.”
This is the only reference to something like NFP I could find in Casti Connubii. The important contrast that Pius XI makes is between “virtuous continence” and “frustrating the marriage act”. Various rhythym methods were in wide use by the end of the 19th century. Surely, Pius XI would have known this. The meaning of this contrast is simple: If you don’t want to get pregnant, don’t have sex. You can’t have your cake and eat it too. Now, that’s not to say that Catholic couples have carte blanche to avoid parenthood indefinitely, at least not without good reason. Weariness of children, satisfaction of selfish desires, and professed inability to remain continent do not qualify as good reasons. What Pius means by “difficulties whether on the part of the mother or on the part of family circumstances”, I do not know.