Quite a good discussion started up in the comments to that post, and it is in response to that discussion that I would like to do a more focused (and hopefully more organized) post about just what, in Catholic thought, constitutes the "moral equation" of any act, and demonstrate that NFP cannot actually be condemned in any element of that equation (whereas contraception can).
Now, I am not actually a moral theologian, and I think there is actually quite a diversity of opinions constituting exactly what defines these three elements, but traditionally an act has been judged as good or bad based on: intention/intended end, moral object, and circumstances/foreseeable consequences.
As I understand the terms, the intent is the end sought in the choice (the "why?"), the moral object is the means or instrumental causes chosen to obtain that end (the type of act itself, the "how," although considered in terms of the object the will chooses and not simply as an externalistic/materialistic description), and the circumstances are relevant conditions usually related to consequences which may be foreseen to result. All three must be moral for the act to be moral.
Now, this may not be entirely traditional, but I would argue that in one sense, at least, the intent is always good. People may be able to formulate proximate intents which are bad, but the Will is only ever drawn by some good (even if only a subjective good). The problem with the intended end, then, can only come in its relation to the means, it seems to me.
Some examples might help explicate the difference, by showing different acts made problematic by each of the three "fonts of morality."
An act problematic in intent, for example, might be taking a recreational hard drug. Let's pretend for a moment there is no problem with the consequences in terms of health-effects. And I think we can assume that, in itself, injecting a chemical into the body is not intrinsically a choice against any natural good. However, the intended end is the meaningless pleasure, the subjective experience the chemical is able to "hack" the brain to obtain. In itself, pleasure is a good, or it would not attract the Will...but the problem here is that in relation to the moral object, this good intended is not intelligible. There is nothing about injecting a chemical that is intelligibly good (or, at least, not as good as the pleasure would indicate), so this act would be illicit, is choosing an evil in that disorder.
An act problematic in moral object could be murdering to satisfy vengeance against a criminal. Again, the intent in itself may be good (the satisfaction of justice), and there may be good consequences from his death that outweigh the bad. But, nevertheless, his death is instrumental to achieving the end sought. The Will is consenting to the evil of his death as a direct means to the end, and as such is choosing an evil. A private individual could not morally do this (though someone acting merely as the "hands" of the State-body in its own self-defense or "medical amputation" might).
Finally, an act that might be problematic in merely the consequences (and there are many like this) might be pulling the trigger on a gun when someone is standing in front of you. Note: this is subtly different than the previous example. In this case, we are assuming you don't intend their death in any sense of the word. You're shooting for some other reason or satisfaction in which their death is not at all instrumental or causal (ie, you obtain it whether they die or not), but are just...blithely willing to accept the consequence of the very high risk of that resulting from you pulling the trigger. This is also bad and equivalently homicide.
Foreseen but unintended consequences, being only indirectly consented to by the will, are different than the other two fonts, however. There can be some bad consequences as long as they are proportionately outweighed by the intended end and/or good consequences (ie, the principle of Double Effect), and we can also never be held responsible for all the remote consequences echoing through history not foreseen.
Now, the condemnation of contraception rests, I think, largely on the first sort of defect: a disordering between intended end and moral object. A pleasure is obtained that is not justified by the type of act chosen, so a subjective good is chosen without its corresponding objective and intelligible good, which amounts to a solipsistic incurvatus in se, a placing of one's Last End in merely a signifier gutted of its transcendent significance. Going through the motions and experiencing sexual activity as good subjectively, but then deliberately excluding the very rational end that makes it objectively good in the first place, making the whole thing arbitrary and absurd, short-circuiting any intelligible meaning of the good therein.
(Sterilization, on the other hand, would be condemned, whether sex followed or not, simply as a mutilation, simply on account of the bad consequence of deliberately damaging fertility. In itself, it is not a sin against chastity, but rather against bodily integrity. However, because double effect allows good ends and consequences to outweigh bad consequences, procedures which sterilize are accepted by the Church if the intended end is some other medical good, just like other mutilations, such as amputations, may be accepted similarly for health reasons.)
Now, I will try to show why NFP cannot be condemned on these grounds, nor on the grounds of any of the three fonts of morality.
First, it must be noted that there are really three sorts of acts which make up the common notion of limiting births with NFP: gaining the knowledge of which days are fertile or not, abstaining on fertile days, and having sex on infertile days. If each of these acts is individually non-controversial, their mere combination in any fashion cannot become suddenly controversial.
The first act, that of seeking knowledge of which days are fertile and infertile...should be pretty obviously seen as fine by everyone. There is never any positive obligation to remain ignorant about anything, sometimes infertility is obvious (as in pregnancy, or when it is perpetual), and the methods used to acquire the information are not of themselves immoral. There is no problem with the moral object of having or seeking this knowledge, no natural good is negated in the instrumental chain of means used to obtain it, there are no evil consequences of simply having the neutral knowledge (which, in itself, can be used to either aid or avoid conception).
The only thing people may object to is the intent. This knowledge is being gained to, usually, avoid pregnancy. However, this is not a bad intent. "Avoiding pregnancy" is not an evil in itself unless we propose that everyone is at all times obligated to be attempting to conceive babies. But obviously that's not true, or we wouldn't allow celibates, and would have to equally condemned married couples who avoid pregnancy by total abstinence as opposed to periodic. There is no positive obligation to be always actualizing a given good (even if married couples do probably have a general obligation to try to actualize it at least once). And as discussed above, contraception is not condemned on the simplistic grounds of "avoiding pregnancy" (which equally applies to celibacy or total abstinence), but on the disorder between the moral object and the intended end.
The next act that makes up NFP is abstaining on fertile days. This, again, should rather obviously be non-controversial. Unless there is some pressing reason that would demand sex on a fertile day, the consequences are neutral or even non-existent (it's hard to speak of consequences for a non-act, and abstinence is just that: it's morally passive). The intent is, again, to avoid pregnancy, which is not a bad intent unless we posit a positive obligation to always conceive or to conceive whenever possible (in which case, as discussed above, we'd have to condemn celibacy and any form of total abstinence too). And the moral object is really, as I just said, a non-object. Abstinence isn't a positive act or reality, it's a passive non-act, as it were. Unless there were a positive obligation to have sex whenever fertile, abstinence cannot be condemned.
The real act that seems to be the "problematic" one in the NFP debate...is the positive act of having sex on the infertile day, as some argue that this is equivalent to contraception (and thus must either be condemned, or use it as evidence of why contraceptive acts should be allowed). However, this is seriously confused.
First, this act cannot be condemned on account of intent. The intended end in having sex that day is, assumably, sexual pleasure or release. In itself, this is not bad, as has been discussed (the only possible evil could come in whether it was ordered properly relative to the act in which it is taken). It would be utterly absurd to claim that the intent of having sex "is to avoid conception." That is not the reason the couple is having sex! That would be absurd and incoherent. (Nor, as I explained in my previous post, do I really think even an actual conception in-itself can "explain" the pleasure or serve as its corresponding intent, even when the couple is trying to get pregnant, given that conception happens later, and non-voluntarily, inside the woman, whereas the objective good will coincide temporally with its subjective good).
But, some might say, it's obviously the reason they are having sex that day. Fair enough, but the question of timing is one of circumstances, not intent or moral object. It is clearly not part of the intent, because as a good article describes: “There is, strictly speaking, no such thing as moving the sexual act from one time to another time. Suppose one wants to say the act is 'moved' from a fertile Monday to an infertile Friday. But one cannot say this. A human act is something that is unrepeatably defined temporally. A sexual act on Monday and a sexual act on Friday are two different acts. The act of abstaining from the sexual act on Monday and of engaging in a sexual act on Friday is not an act of transferring the sexual act from Monday to Friday, because it is a logical impossibility, strictly speaking, to transfer a specific act. The unrepeatable Monday-sexual-act cannot be moved to Friday any more than one can move the Monday itself to Friday.”
Likewise, the timing to avoid conception is also not part of the moral object either. There is nothing instrumental about the avoidance of conception or the timing of the act in terms of the couple achieving pleasure. They would achieve that intended end regardless of whether the day was fertile or not. The act chosen to obtain it, if "considered in a vacuum," is the same act (the moral object could be phrased something like: stimulation of the genitals culminating in ejaculation in the vagina) whether the day is fertile or not. The infertility of the day is not part of the causal chain of means chosen to achieve the intended end of the pleasure, so it is not part of the moral object.
Thus, the timing/infertility is relevant only as a circumstance. Some might say that, based on the logic of condemning contraception, any sex during infertility should be condemned because "avoiding conception" is a consequence. However, besides the fact that we have already established that contraception is not condemned due to some supposed bad consequence of avoiding conception but rather a disorder in reason between intended end and the means chosen to achieve it...this is also simply absurd. Just like with the intent, "avoiding conception" can't be said to result from the sex act! Having sex isn't causal when it comes to the non-conception which, besides being a metaphysical zero in itself, would happen that day whether the couple had sex or not!! So it cannot logically be called a "consequence" of the sex.
The only way, then, that NFP could be condemned is if it were established that the same sort of disorder between intended end and chosen means existed as exists in contraception. Some people might jump to say, "But clearly! The intelligible good which rationally relates the pleasure to the act is conception, but that is not possible on an infertile day!" However, this is a categorical error. As has already been established two paragraphs above, the infertility of the day is not part of the moral object. It is a circumstance, but it is not part of the direct chain of causes chosen to obtain the pleasure. And, of course: infertility that already exists cannot be said to be directly part of the free choice of an act after-the-fact.
Therefore, the moral object can only be judged in itself. Only the type of act can be judged relative to the intended end, since we have already established that this circumstance of infertility is not part of the moral object and cannot logically be said to actively effect the consequences one way or another (no conception happens whether sex occurs or not!) Since the infertility is neither part of moral object nor intent, it certainly can't be part of the relationship between them. And is the act proposed, the moral object of "stimulation of the genitals culminating in ejaculation in the vagina," ordered properly relative to sexual pleasure? Yes. Clearly. If it wasn't, we'd have to condemn sex period.
Given that our society has a consequentialist view of morality generally (and given a lot of bad explication of this teaching even within the Church)...it may seem odd to people that the intelligible good of procreation can confer rational order or significance or meaning on the pleasure of the act of ejaculation in a vagina even when actual procreation is impossible or unlikely, even when infertility is known to exist. However, that is the internally consistent conclusion of our moral system.
Because the infertility is not a part of the moral object in itself (inasmuch as it is not instrumental to the pleasure sought), and because it also can't be said to cause a bad consequence (inasmuch as no conception would occur that day whether sex takes place or not, and inasmuch as "avoiding conception" in itself is really not an intrinsic evil either, if only because it is a non-entity)...we can only judge the proper ordering of the pleasure relative to the type of act which occurs.
And the type of act to which sexual pleasure intelligibly corresponds is the type of act which sometimes leads to conceptions, and indeed for that very reason. But, for the reasons explained above, actual conception or fertility is not required every time for a proper ordering of the Will, exactly because the ordering of the end can only ever be judged relative to the type of act, to the sort of moral object (of which the ultimately inconsequential circumstance of infertility is not an instrumental part). There is no defect in any of the three fonts of morality with NFP, and yet there is with contraception (exactly because contraceptive "sex" is a different sort of act, is not of the type given rational order by procreation).