From this post, it seems that practicing NFP is a sin, unless you're in one of these four situations:
"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 waste-line 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 preclude 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."
Honestly, there are very few people in modern America that would fit into any of these categories. There may be a few people in category 1 and a tiny amount of people in category 3, but I see practically no present-day Americans that fit into categories 2 and 4.
But from reading other sources, it seems that NFP is fine no matter when and these conditions are rarely, if ever, mentioned.
There are a couple of things to keep in mind with the Church’s teachings on NFP.
First of all, she has never laid out occasions where NFP would or would not be sinful. The four cases laid out by the previous poster are his opinion on when NFP is permissable. Now, I don’t doubt that these were reached through careful research and prayerful discernment, but they are still opinion.
The Church is much more general, because, honestly, this is truly a case that will vary family to family. Whereas some familes, mothers, fathers and children, will do well with children spaced 10-12 months apart, others will do better with closer to 24 months. And there’s nothing wrong with either scenario, provided every family seeks to do God’s will for them. And that Will will vary from family to family.
It’s important to note, that the term “grave reasons” is used often when speaking of NFP. HOWEVER, the term “grave” when speaking of the reasons a family might seek to use NFP is only used in the Pauline translation of Humane Vitae. The official Vatican translation uses the term “just” reasons which does change the tenor of the discussion. And what is just for a family is going to vary family to family due to personalities and circumstances.
The primary duty of parents is to educate their children. And the way each family needs to educate their children will vary. They should pursue the methods and tools they need to in order to properly educate their children.
But from reading other sources, it seems that NFP is fine no matter when and these conditions are rarely, if ever, mentioned.
I got out of teaching NFP when the NFP industry started teaching that NFP can be used for any reason whatsoever. The magisterial documents all mention grave or serious reasons. Pope Pius XII addressed this in his Address to the Italian Midwives, which specifically mentions the four types of scenarios I outlined:
“Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called ‘indications,’ may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life. From this it follows that the observance of the natural sterile periods may be lawful, from the moral viewpoint: and it is lawful in the conditions mentioned. If, however, according to a reasonable and equitable judgment, there are no such grave reasons either personal or deriving from exterior circumstances, the will to avoid the fecundity of their union, while continuing to satisfy to the full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles.”