I'll repeat this from the other thread.
Pius XI taught that TWO principles applied in ascertaining the moral qualities of the marital act:
1) that the "natural power" or "inherent force" of the action cannot be violated (artificial birth control, Onanism, etc.)
AND
2) that the primary end or purpose of marriage cannot be subordinated to the secondary ends.
Pius XII in the Allocution DROPPED the second condition or consideration enitrely. I'll go fetch my juxtaposition of Pius XI and Pius XII's paraphrase of Pius XI.
NFP is different from ABC in that it avoids violating principle #1. In other words, it's not INTRINSICALLY immoral, whereas ABC IS because by its very nature it inherently violates the first principle.
But just because something isn't intrinisically immoral doesn't mean it can't be immoral for other reasons, i.e. because it's FORMALLY immoral, i.e. due to the intent.
Pius XII argued that since one may abstain from marital relations, it's OK to abstain from marital relations ONLY during fertile times. His reasoning is just plain wrong. That does not necessarily follow. In abstaining altogether, the couple actually tacitly respects the primary ends of marriage.
HOWEVER ... when a couple abstains from the marital act only during fertile periods, they are clearly trying to exercise the marital act for the secondary ends while DELIBERATELY ATTEMPTING TO EXCLUDE REALIZATION OF THE PRIMARY END.
If that isn't subordinating the primary end to the secondary ends (which Pius XI condemned), then I don't know what is. Ambrose has not yet come close to explaining how one can engage in selective abstinence during fertile periods and NOT be doing exactly what Pius XI condemned. I'm all ears.
And just because Pius XII put a condition of "serious reasons" on NFP doesn't mean that his principles didn't open the floodgates on NFP. Because we're just talking about various interpretations of "serious". When Pius XII lists among "serious" reasons things like "eugenic" and "social" and "economic" considerations, that clearly opens the floodgates. Serious doesn't have to mean "grave" or "dire"; it only serves as a contrast to "light" or "trivial". So, based on Pius XII, if I'm having some trouble paying my bills or I'm psychologically worn down by having lots of loud children, or I'm older and have increased risk for having a child with Down syndrome ... all that could easily qualify me for NFP. Or we're having relationship problems, so it would be unhealthy for a child to be in this environment. There's no clear, hard, solid line here from the Pius XII teaching.
I'll dig up my contrasting Pius XI vs. Pius XII quotes.
PS -- Ibranyi got it wrong for the same reasons I'm citing here, his failure to distinguish betwen #1 and #2 as listed above. Ibranyi conflated the two considerations.