Here's the way I explained it on an old Catholic forum:
NFP CAN BE and often IS used, and even TAUGHT, in a sinful manner. Yet it is NOT and can NEVER be inherently sinful as artificial contraception is so.
NFP itself is NOT inherently sinful, and anyone claiming otherwise is not only misrepresenting post-conciliar but also pre-conciliar Catholic moral theology.
To say that NFP is ALWAYS sinful is just as wrong as to say that NFP is NEVER sinful.
If my "INTENTION" is to bring home enough money to feed my family, that is a good thing. I may get a job, bring home my salary, and feed my children. The job is a licit way to achieve a licit thing.
On the other hand, I could rob a bank and get enough money to feed my family for a whole year. That is an illicit way of achieving a licit good thing.
The same is true for child spacing. If my children would literally starve if my wife were to get pregnant, it is morally licit to space children until I could afford to feed them.
NFP would be a morally licit way to acieve this necessity.
But artificial birth control is intrinsically evil. It can never be morally licit to have recourse to artificiaql contraception.
So to answer your question, the INTENTION in having recourse to EITHER artificial family planning OR "natural" family planning could be illicit or licit. One may be sinsul, one may not.
However, the method itself, in the case of artificial birth control, is intrinsically illicit, i.e. regardless of intent is it gravely sinful.
However, NFP itself is morally neutral. It becomes morally illicit when the intention itself is illicit.
4 main reasons for having recourse to NFP.
1--Physical/ mental health---a pregnancy could kill you or so physically impair you as to prevent your fulfillment of your duties in your state in life---NOT because of a widening wasteline or drooping skin! Or psychological health, i.e., mom would literally have a nervous breakdown if she became pregnant---not because she "just couldn't stand being home with the little kids all day without the personal fulfillment of her professional job..."
2--Financial constraints---your child will starve if you have another. Wanting a bigger house or designer SUV just does not cut it!
3--work on the mission fields by one or both spouses that would proclude having children temporarily
4--active persecution or war---i.e., you or your child likely to die by coercive abortion, in concentration camp, in acts of war, etc.
Clearly we say these reasons must be SERIOUS, not trivial. Only the couple and their confessor can truly decide what truly constitutes grave reason.
We've had couples sit through my talk on this subject and literally say, "Gee, we thought we were being good Catholics just for deciding to use NFP. Now we realize we don't even have grounds for recourse to NFP," then tell us a month or two later they're pregnant.
NFP vs Contraception
Spacing children may be a desirable goal that does not violate God's laws in certain serious situations such as those outlined above. But the means of achieving the goal differ.
One is intrinsically evil (abortion, abortifacient contraception, barrier methods, sterilization) while one is morally neutral (Natural Family Planning.
In one, an act is performed (sex) but its natural outcome is artificially foiled.
In the other, no act is performed (simple abstinence during fertile times) so there IS no act, therefore the practice is morally neutral.
It is then the intention of using NFP that constitutes its relative moral licitness or illicitness.
If NFP is used in a selfish manner, it too can be sinful.
If it is used only in grave circumstances, it is not sinful.
The difference is real.
Dieting (decreasing caloric intake, the "act" of NOT eating) is a moral and responsible means of losing weight to maintain the body's health.
Bulimia (the ACT of eating, them vomiting) is rightly called an eating DISORDER.
An ACT is performed (eating in this case) and its natural outcome (nutrition) is foiled by expelling the food from the body.
Likewise contraception is a disorder. An ACT is performed (sex) and its natural outcome (procreation) is foiled by expelling the sperm or egg or both (abortifacient contraceptives) from the body.
Contraception is to NFP what Bulimia is to dieting.
But just as dieting can be misused (anorexia) so too can NFP be misused in a sinful manner
So much for “God will provide” if He gives us more children. The lack of faith of Catholics is astounding.
Preventing conception is preventing conception.
It’s just a matter of how you can justify it.