Some thoughts:
Both NFP (assuming it isn’t being used to become pregnant) and ABC accomplish the same thing: they reduce the rate of pregnancy. Is it wrong to reduce the rate of pregnancy?
If it is, then not only would NFP and ABC be wrong, but it would also be wrong for a husband and wife to practice any form of long-term abstinence (for an extended spiritual purpose: see 1 Cor. 7; or because one is away on a mission trip for a long time), as that guarantees a 100% reduction in the pregnancy rate (barring divine intervention). I haven’t seen anyone seriously arguing that, and I certainly wouldn’t argue that, so I will assume that in and of itself, a reduction in the rate of pregnancy is not an evil.
If that particular effect isn’t intrinsically immoral, then we must compare the different causes. Is it immoral for a couple to decide for health reasons to practice permanent abstinence? In this case the reduction of the pregnancy rate isn’t incidental but is the desired effect. Note that this is not NFP but total abstinence. There might be objections to this practice for other reasons, but again I haven’t seen any for the reason that they are trying to avoid pregnancy.
If it is morally permissible for a couple to practice total abstinence for the purpose of avoiding pregnancy in some cases (such as with health issues), then I have demonstrated that an intention to avoid pregnancy isn’t intrinsically evil. A couple could sin in this way, but that would be due to selfishness or some other evil motivation and not the avoidance of pregnancy itself.
Now to reintroduce NFP: Assuming the absence of selfishness, is there anything incompatible with the full meaning of sexual intercourse and a desire to avoid pregnancy? Does that desire – even if it isn’t acted on – harm either its unitive or procreative aspect? Is there a contradiction between the act and the desire? Those are very important questions. To be honest, I cannot answer them, but I would like to present a possible response by way of analogy:
In the Garden of Gethsemane, Jesus prayed, “Father, take this cup from me. But not my will, but yours.” That would seem to indicate that Jesus had some sort of desire to not be crucified, but his desire did not put him in contradiction with the will of the Father.
I think it is possible for a couple to be justified in practicing NFP and abstaining from those days that are the most fertile. Merely desiring or intending to avoid pregnancy isn’t wrong, neither is abstaining to avoid it. There isn’t a contradiction present between the intention and the abstention. The only question that remains in my mind is whether there is a contradiction present between simultaneously occurring desire and sexual intercourse.
If there is a contradiction, then ABC is in the same situation, and we can even more confidently denounce it. Both NFP and ABC have the same desired outcome. Is there any substantial difference between the two? In this debate, it is frequently objected that if ABC is wrong, then NFP isn’t any better because they both have the same intent. However, I hope I have shown how neither the intent nor what is considered a successful outcome are intrinsically immoral. If ABC is wrong and NFP isn’t, it has to be due to a difference in means.
Again assuming an absence of selfishness, there is no aspect of NFP that is wrong (except for the aforementioned possibility of internal contradiction): sexual intercourse isn’t wrong, abstaining isn’t wrong, avoiding pregnancy isn’t wrong. All that NFP does to achieve its goal is combine those three elements with the naturally occurring infertility of a woman. The ABC difference? It adds (usually replaces abstaining) an externally forced infertility to the sexual act.
NFP passively allows the infertility to exist, while remaining at least symbolically fertile. ABC causes the infertility to exist, and there is no symbolism of fertility.
Is that a significant difference?