Mental prioritization isn't the same thing as proper subordination, which in the context of CC and the natural law follows from some act. I may prioritize the taste of a meal over its nutritive qualities, and I may be motivated to eat the meal because of its smell or appearance rather than out of a Aristotelian appreciation for its participation in an intricate network of causality and ends. And in so doing I would never be subordinating the primary end of consuming the meal to a secondary end. That's just boilerplate human behavior. Now, if I eat the meal and then go to the bathroom and purge, I'm frustrating the primary end of nutritive consumption and subordinating it to secondary ends.
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So if you want to argue that there is devious, even sinful intent in having marital relations only in sterile periods, let's argue that, but let's just be really clear that such an argument has nothing to do with the natural law, frustration, or the subordination of ends.
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Condoms. Birth Control Pills. And so on. Those all directly frustrate (not avoid) the power of the act in a way that makes the realization of its primary end virtually impossible. That is true subordination. What you're describing is, at worst, an interior disposition of "not wanting to conceive right now." Not a perversion of the act itself, which is the problem with birth control.
You open this argument with stating that mental prioritization is not the same as subordination ... but then go on to conflate the two. Obviously, the couple need not be thinking, every single time they have relations, "Yes, indeed, the MAIN reason we want to do this is to conceive a child and only secondarily do we want these other things." Some Church Fathers would argue that this would be venially sinful, but that's a digression. In any case, no, obviously mental prioritization is NOT the same thing as subordination. Indeed, if a couple were to engage in marital relations primarily because they gave into passion or desire and weren't really consciously thinking about having children, this is not tantamount to subordination.
But what part of this do you not understand? WITH NFP A COUPLE IS ACTIVELY ATTEMPTING TO THWART THE PRIMARY END OF MARITAL RELATIONS SO THAT THEY CAN ENJOY THE SECONDARY END WITHOUT THE POSSIBLE BURDENS THAT MIGHT COME WITH THE PRIMARY.
Nor is the thought of "not wanting to conceive right now" anywhere near the same thing as, again, read my lips, actively attempting to have marital relations ONLY during infertile periods so as to thwart the secondary ends. So that's absolutely false that what I"m describing is "at worst, an interior disposition of 'not wanting to conceive right now'". That's utter nonsense. I am speaking about nothing of the sort. Indeed, a married person might even think, after having engaged in marital relations during a fertile period, "Boy I hope we don't get pregnant right now; we're really having trouble with money lately." Again, that is not even close to the same thing that I am describing, much less "at worst". What I'm talking about is the FORMAL INTENT to frustrate the primary end. "Hey, we're going to deliberately go about having marital relations ONLY during infertile periods so that we can prevent pregnancy."
I've ALREADY CONCEDED that NFP does not involve a perversion of the act itself. So why are you arguing this? I'm talking about the FORMAL INTENT to thwart the primary end while enjoy the secondary. That is subordination. Period.