Now, of course the primary ends must be subordinated to the secondary ends-- we agree on this. My point is that Casti Conubii regards this subordination as manifest when the act occurs naturally. There is no distinction to be had between the intrinsic preservation of the act and the ordering of ends. They are one and the same. If the ends are properly ordered, it means the act is preserved, and if the act is preserved, it means the ends are ordered.
I think you are skipping over the crucial distinction between intrinsic and extrinsic. Casti Conubii, when it teaches on marital relations, concerns itself very squarely with what is intrinsic to the act. Motives, intentions, and so forth are all extrinsic. Even if one is motivated insufficiently (for lawful use of periodic continence), the sin committed is not one against nature.
With all due respect, this is complete nonsense.
First, I suspect it was just a typo when you wrote that "of course the primary ends must be subordinated to the secondary ends". Presumably you mean the other way around.
[Mith:] "Motives, intentions, and so forth are all extrinsic. Even if one is motivated insufficiently ..., the sin committed is not one against nature."
So what if the sin committed isn't against nature? Of course, to a degree, any subordination of the primary end is "against nature" to some extent, since the naturally-intended end of the act is procreation. But, the term contra naturam
isn't applied strictly this way. So what? Unmarried couples who fornicate also do not sin against nature (in this sense), but they sin mortally nonetheless.
Certainly the relations that takes place in the context of NFP are not against nature.
But I've already addressed this. We're human beings, not animals, and our intentions clearly inform the morality of our actions. Morality is primarily in the intellect and will. Our Lord taught precisely this when He said that the man who lusts after a woman commits adultery in his heart. If I pocket a $100 bill that belongs to someone else, thinking mistakenly that it's mine, I commit no sin ... even though objectively (intrinsically speaking) I have committed theft. Conversely, if I pocket a $100 bill that in fact belongs to me, thinking mistakenly that it belongs to someone else, I commit a grave sin. Motives have everything to do with the morality of our actions.
Here are the ends that a couple might intend or seek or pursue in marital relations:
II) Secondary Ends (mutal affection, allaying of concupiscence, etc.)
III) Carnal Pleasure
There are many venial-sin degrees of subordination, where people might be "insufficiently motivated" to use your term.
It's a rare thing for a couple to have perfect motivation which would be: "We are doing this primarily to have children, but in doing this together it also happens that we grow closer to each other, and the pleasure is part of that closeness and the act of procreation itself." That doesn't happen too often, only in the case of the great saints, such as Sts. Joachim and Ann in conceiving Our Lady.
Most couples are primarily thinking of end II or even end III. Depending on the degree of disorder, we're talking about imperfection or varying degrees of venial sin. But there's a whole continuum of imperfection and even sin here.
But even when a couple are entirely focused on III and give I and II no thought whatsoever, perhaps considering II a consequence of III, there isn't grave sin there.
Why? Because there's still the principled virtual intention present in the intellect and will by virtue of the fact that the couple know they're married and lawfully engaging in marital relations, the primary end of which is to conceive children. Similarly, a priest can be said to have a virtual intention when consecrating during Mass. He doesn't have to explicitly think the proposition: "I hereby intend to consecrate this bread." He has virtual intention because he knows in his intellect that he's offering Mass, during which transubstantiation happens as a result of his saying the words of consecration. Same kind of thing happens with a married couple in the heat of passion when they're not giving the primary end a second thought. Now, a husband could even, for example, think, "boy, I really hope she doesn't get pregnant, because that would be hard for us financially right now" -- and not commit a mortal sin. This would be a graver degree of venial sin, but in his intellect he still acknowledges ... and does not actively prevent ... the primary end, even if emotionally he doesn't want it. Similarly, I could get up on Sunday morning really tired and say, "I really don't feel like going to Mass." ... but then you go anyway because in your intellect and will you intend to meet your obligation, even if you don't particularly like doing it that day. In fact, there could be greater merit in that than if you go because you are just loving the experience. BECAUSE human actions are informed of their morality in terms of the intellect and will, rather than the emotions.
Even if an infertile couple have relations or a fertile couple have relations in an infertile time, they still intellectually acknowledge and accept in principle that the primary end of the activity is procreation ... even if that end is not possible to ACHIEVE or to actualize for them. You could have a couple intending procreation but not have it work out for them at any given time. They still INTENDED the end, at least in principle, even if they could not attain it in actu
None of these types of examples you cited has any bearing on the morality of NFP. With NFP, unlike all these scenarios, where the motivation or intention is either imperfect, present only virtually, or partially disordered (to the extent of venial sin), the couple actively rejects and seeks/intends to thwart the primary end. This is a completely different animal than these other situations. You're rejecting it in your intellect and will and actively attempting to exclude it. That entails a mortally sinful rejection of the primary end. You are seeking the secondary ends (or usually just mere carnal pleasure) TO THE POSITIVE EXCLUSION of the primary. That's mortal sin. It's one thing for the primary intention to be lacking (to a degree) negatively, as long as it's present virtually (in the intellect and will), but quite another thing when a couple intends to exclude it completely.
And that's where we address the 1880 Holy Office ruling. Recall that rhythm in the day meant a lower likelihood of conception and not a complete elimination of the possibility. Consequently, it's a venial-sin degree of insubordination that the Holy Office stated could be tolerated (very carefully, under strict circumstances, so as never to give the impression of allowing a lesser evil in the prevention of a greater) ... a matter to be handled very delicately by Confessors in the case of someone who might otherwise practice Onanism. If it were not sinful to a degree, there would be no need for the advice of caution and delicacy in allowing the practice. But lessening the likelihood of conception entails a PARTIAL rather than COMPLETE (therefore a venially-sinful vs. mortally-sinful) subordination of the primary end. With NFP we're talking about its elimination.