It all boils down to one central feature: NFP isn’t artificial.
Still, that simple fact won’t silence most objectors. “If you’re trying not to get pregnant, then isn’t that basically the same as using a condom? The result is the same; what’s the difference?”
There are surely selfish ways to employ natural family planning—just as selfish as using birth control, condoms, etc. Most good things, in excess, can be somehow perverted.
Instead of focusing on NFP-as-failed, let’s give a bit of attention to the natural dissimilarities between NFP and contraception. The clincher comes when we take sexual intercourse as an integral act: namely, one that involves not just physical processes, but also intentions. Neither alone makes up the act of sex; and both are separable from one another in some way. (E.g., a rape victim hardly “has sex with” her aggressor. Equally, one who lusts after his friend’s wife isn’t blameless, just because he didn’t “do” anything.) In short, it’s possible to have simulated intercourse that reflects certain aspects of—but isn’t the same as—the real thing.
Artificial contraceptives—when used for this purpose—degrade sex from “real” to “simulated.” In other words, they remove or mitigate a feature of the action, i.e., the organic, physical complementarity of male and female, leaving behind only part of the “material” needed for sexual intercourse. Furthermore, using contraceptives means not only that a couple wishes to avoid pregnancy, but that they intentionally mar the act to achieve this end.
On the other hand, NFP avoids both of these pitfalls. It leaves intact the full biological “material” of sexual intercourse, and doesn’t require a couple to prioritize intentions over the act, itself. Avoiding pregnancy by gauging natural fertility cycles doesn’t demand a separate intention (e.g., putting on a condom) aimed at rendering the act fruitless. Rather, any intention to avoid pregnancy is subjected to a freely engaged in, physical action that is known to be naturally fruitless at certain times.