NFP is a form of birth control. There is nothing inherently wrong with birth control; it allows is to plan our family so that we can support eachother in the best way. It is artificial birth control that is sinful (removing the procreative aspect). NFP does not remove the procreative aspect; it only involves periodic abstinence.
I found some answers in the AAA forum. This has been asked many many times. Here are the search results. There are two in particular that I found helpful:
What is the moral difference between contraception by artificial means and NFP?
[quote=Jim Blackburn]“The regulation of births represents one of the aspects of responsible fatherhood and motherhood” (Catechism, 2399). Contraception and natural family planning (NFP) are common methods used to regulate births, however, there is a big difference in their respective morality – while the ends may be the same, the means are very different. Contraception attempts to close the marital act to procreation while NFP does not. Instead, NFP observes abstinence during fertile times – marital activity, however, is never closed to procreation.
The catechism explains it this way: “Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible is intrinsically evil: Thus the innate LANGUAGE that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory LANGUAGE, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. . . . The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle . . . involves in the final analysis two irreconcilable concepts of the human person and of human sexuality” (CCC 2370).
Why is NFP okay?
[quote=Michelle Arnold]NFP, in and of itself, is morally neutral because there is nothing inherently evil about refraining from sexual relations, even with the knowledge that doing so will prevent pregnancy. NFP can be used in a selfish way, such as with the birth-control mentality you so vividly describe, but it can also be used in a prudent manner if the couple has just cause to delay future pregnancies.
On the other hand, artificial birth control allows the couple to indulge in the sexual relations while blocking its natural result of pregnancy. Artificial birth control is like saying to God, “No thanks, God, I don’t want the trouble of raising kids … but I’ll have sex whenever I want to and prevent its intended purpose of pregnancy!”
Hope that helps!