The “Rhythm in Marriage and Christian Morality by Fr. N. Orville Griese, S.T.D, J.C.L.,1944, Pages 39-41
The Encyclical "Casti Connubii"
Quite a few contemporary theologians are of the opinion that His Holiness, Pope Pius XI expresses approval of the "safe period" practice in his encyclical on "Chaste Wedlock." The passage in question is the following: speaking of the evil use of matrimony, the Holy Father says that the husband or wife is not guilty of sin "when for a grave cause he or she reluctantly allows the perversion of the right order," and he adds:
“Nor are those considered as acting against nature who in the married state use their right in the proper manner, although on account of natural reasons either of time or of certain defects, new life cannot be brought forth."
The text would seem to indicate that the Holy Father is speaking of those who are physically sterile ("certain defects") or who have passed the menopause ("reasons ... of time"); those are times "when new life cannot be brought forth” In the case of "Oginists," new life can be brought forth. In fact, that is precisely the reason why such a complicated practice is adopted.
A study of the context leads us to a similar conclusion. In the same section of the encyclical, the Holy Father mentions that the Church "well understands and clearly appreciates all that is said regarding the health of the mother and the danger to her life;" (p. 92) that he is "deeply touched by the sufferings of those parents who, in extreme want, experience great difficulty in rearing their children." (p. 93). He likewise refers to those who "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 circuмstances." (p. 91). It would seem that the mention of the "safe period" method would be fitting in connection with one of the above phrases if it was to be mentioned at all. Instead of suggesting a system of voluntary sterility, however, the Holy Father reminds the mothers that "God will assuredly repay them (her) in a measure full to overflowing;" (p. 92) he reminds both spouses that "there are no possible circuмstances in which husband and wife cannot, strengthened by the grace of God, fulfill faithfully their duties and preserve in wedlock their chastity unspotted;” (p. 93) that "any use whatsoever of matrimony exercised in such a way that the act a deliberately frustrated in its natural power to generate life, is an offense against the law of Gad and of nature…”
We must admit that if the Holy Father had any intention of approving of any system such as the “safe period" method, he does it in most obscure language. On the other hand, the meaning of the disputed text is clear and intelligible if we do not try to force a reference to the "safe period" in between the lines.