Pope Pius XII said we need “grave” reasons, as did Pope Paul VI. Pope John Paul II’s catechism said we need “just” reasons, while his Pontifical Council for the Family used both “serious” and “proportionately serious.” So I think I can confidently say that a motive for resorting to NFP must be grave, seeing as serious and grave are, in morality, basically the same thing. The problem is, beyond financial solvency and health (physical, mental, emotional, spiritual), we are given little guidance as to what constitutes serious reasons.
I have a particular issue in mind, and while I know you will probably hesitate to pronounce on specifics, I figured I would ask your opinion. My sister plans on being a doctor and is under the impression that she can stop having children whenever she feels like it because her job will always be a reason not to have children. But if her job is not necessary to support the family, can she opt to use NFP solely in order to be able to continue working? That seems like preventing more children in order to keep up with a really demanding hobby or buy another boat. Since I know her chosen profession is actually a very valuable one that might factor in to the decision, I think the most useful question for all cases would be, “In general, may one use NFP to space births solely for the purpose of being able to pursue a chosen profession?”