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.
Another thing that’s hard for me to understand about NFP is this: It’s wrong to separate sex from procreation, but that seems to be exactly what you are doing when you practice NFP. I don’t see how practicing it makes you more open to life when it is actually more effective than most artificial contraception.
On another note, I see on the post after yours to me that you are going to have a baby. Congrats!