I don't think your metaphor works for the following reason: What if our bodies were designed in such a way that we only digested our food on some occassions but there were other times when our digestive systems simply ignored any food we ate. In order to stay skinny, what if you deliberately ate only during the times when your body did not digest the food, but refused to eat whenever your body would digest the food? This would not really be equivalent to "dieting"; it would be a lot more equivalent to "bulimia." In both cases you are frustrating the natural purpose of the act.
I also disagree with the moral theology textbook that was posted earlier when it said that the moral issue revolved around abstinence with regard to issues of justice towards society. Clearly a couple can abstain entirely from marital relations if they wish to do so and are in agreement. If a grave reason existed which motivated a couple to avoid conception, they could abstain from marital relations for as long as they wished to avoid conception, and there would be no moral issue whatsoever.
It becomes morally tricky when the couple abstain only part of the time, while continuing to engage in marital relations at other times, but without any intention of fulfilling the primary purpose of those acts. The problem is not abstaining; the problem is having sex while at the same time deliberately frustrating the purpose for which the sex act was designed by God.
At the same time, one can clearly see that there is a significant difference between artificial contraception and NFP. In the former case the act itself as well as the intention are both inherently wrong. In the latter case, the acts themselves either of having marital relations or abstaining from marital relations are not inherently immoral, but the intention can make them immoral.
As a comparison, when one tells a deliberate lie, then that is a sin which could be a mortal sin depending on the gravity of the situation. The words themselves are false, and the intention is to deceive the listener. One can also tell words that are true, but which are equally intended to deceive the listener. One would still be guilty of a sin of deception.
Another comparison might be to the priest offering the consecration. If he uses bran muffins, then the matter is invalid and the entire consecration is invalid, no matter what his intention might be, although one would have to question the intentions of any priest who would commit such a sacrilege. Another priest might use perfectly valid bread as matter, but he has an intention directly contrary to the presumed intention of consecrating the host. His consecration would also be invalid.
I agree. Justice towards society is one issue to take into consideration, but it certainly is not the only nor the primary consideration.
As far as the contraception and bulimia comparison goes, it might not be a perfect analogy, but it IS one that common people grasp. I think I first heard that analogy on a Scott Hahn tape.
Another analogy is the need to support a family.
Supporting my family is a moral good. If I need to support my family, and it takes $25,000 to do so, I can get a job and earn that income to achieve that good end.
On the other hand, I can hold up a bank, grab $25,000, and still acieve the moral good of supporting my family.
Obviously, the MEANS of achieving this moral good are not equal. Thus the argument that since NFP and contraception both achieve the same end, they are morally equivalent, is a logical fallacy. The means of achieving the ends are diametrically opposed.
In our analogy, if our child would be forcefully aborted by an evil regime, it would be a moral good to postpone pregnancy until such time as we can assure the safety of our baby. Both NFP and artificial birth control can achieve this end. But like working for a living versus robbery, or dieting versus bulimia, the means of achieving the end are NOT morally equivalent.