Since “Anonymous” told me to consult some priests and theologians, I did. John and Sheila Kippley are two of the top Natural Family Planning experts and writers out there. Individually and toogether, they’ve written several books on NFP, including “the” guide to sympto-thermal (they were the authors of the official Couple to Couple League book until they started their own school). Sheila has posted here a couple times.
Basically, what he said echoes what I’ve already said, and actually anticipates some of “Anonymous’s” assertions, but here it is:
Sheila forwarded to me your request for some theological input regarding your anonymous interlocutor who says we don’t have to listen to the Popes unless they are speaking ex cathedra. I use the technical term rather than “infallibly,” for the teaching of Casti Connubii and Humanae Vitae is, in the opinion of many, both infallible in their teaching and put forth as infallible teaching by reason of transmitting the unfailing Tradition of the Church. But there is also widespread agreement that it has not been stated ex cathedra.
In response to your correspondent, you will want to refer to Vatican II, Lumen Gentium (LG), n. 25. (LG is the Dogmatic Constitution on the Church, the first of the Vat ii documents.) The second paragraph of that section says it all.
In my book, Sex and the Marriage Covenant, Chapter 7, “Forming a Correct Conscience,” I have applied the teaching of LG 25 to Humanae Vitae. As the late and great Msgr. William B. Smith told me, “John, you have demonstrated ad nauseam the mind and will of the Popes on this matter.”
We might ask, “Why haven’t the Popes declared this issue ex cathedra?” We need to keep in mind that almost all the moral teaching of the Church is done via the ordinary magisterium of the Church. In my opinion, if the Pope were to teach the doctrine of marital non-contraception ex cathedra, that might have the effect of undermining the ordinary magisterium. The dissenters on other issues would say, “See, the Pope taught ex cathedra on contraception but hasn’t done so on issue X.
Therefore issue X is up for grabs.” [note that “Anonymous” made this very assertion: that JPII condemned abortion ex cathedra, so, by extension, contraception must be OK, proving John Kippley’s point.]
Two chapters of SMC are currently at our website in full. Chapter 4, “Holy Communion: Eucharistic and Marital” (first written as an article that appeared in 1967, 15 months before Humanae Vitae); and Chapter 12 dealing with the Repentant Sterilized Couple.