I’m not sure if this has already been addressed in this thread. NFP is allowed by the Church, but only for a “serious reason”. If no serious reason justifies it, it is morally sinful. The problem with NFP today, as with so many other things, is that it is being abused. Some Churches actually have NFP classes that they encourage people to attend! But NFP is only allowed for a serious reason. For example: I know a couple who has 6 children. The mother now has some kind of problem. If she becomes pregnant it is very likely that she will die. That is a good example of a serious reason. But if no serious reason exists, NFP is not allowed.
I am going to quote a portion of a talk that Pope Pius XII gave, dealing with NFP (as well as a link to the entire talk); and also a portion of Humanae Vitae. Both of these tell us that there must be a “gave reason” for married couples to obstain with the explicit intent of not having children.
Pope Pius XII: "The reason is that marriage obliges the partners to a state of life, which even as it confers certain rights so it also imposes the accomplishment of a positive work * concerning the state itself. In such a case, the general principle may be applied that a positive action may be omitted if
grave motives, independent of the good will of those who are obliged to perform it, show that its performance is inopportune… The matrimonial contract, which confers on the married couple the right to satisfy the inclination of nature, constitutes them in a state of life, namely, the matrimonial state. …to embrace the matrimonial state, to use continually the faculty proper to such a state and lawful only therein, and, at the same time,
to avoid its primary duty [having children] without a grave reason, ***would be a sin against the very nature of married life. **
"Serious motives … may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life… 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 tile full their sensuality, can only be the result of a false appreciation of life and of motives foreign to sound ethical principles."
Link to the entire talk by the Pope:
catholicculture.org/docs/doc_view.cfm?recnum=3462
In the following quote, we see that Humanae Vitae also says that “serious motives” must be present to “space our births”, for it to be “licit”.
"If, then, there are serious motives to space out births, which derive from the physical or psychological conditions of husband and wife, or from external conditions, the Church teaches that it is then licit to take into account the natural rhythms immanent in the generative functions… [Pope Paul VI, *Humanae Vitae 16]
So, while NFP is allowed, it is only allowed for “grave reasons”, and not as a substitute for birth control.