Must a couple have "grave reason" to use NFP?
My husband and I have had two children in three years and have decided to use NFP until the younger child is at least potty-trained, which we estimate may be two to three years. Because we are young, healthy, and have a number of potential childbearing years ahead of us, we do plan to return to the providential method at that time, but feel overwhelmed at the thought of three children under the age of five.
My husband's brother and his wife, a very traditionally Catholic couple, maintain that we do not have adequate reason to use NFP. They, who have six children fourteen and under and are expecting seven and eight (twins), maintain that a couple must have "grave reason," which they define as life-or-death, or the couple should leave the number and spacing of children to God.
I suppose we shouldn't have discussed our decision with them in the first place, but now that we have, how do we respond? I certainly cannot tell them that it has been our observance of their own difficulties in raising so many young children that has influenced our decision to put off having more ourselves!
The Church does not use the word grave in its formulation of the allowance of couples to use natural family planning (NFP). Humanae Vitae, in the Vatican's English translation, uses serious (a lower standard than grave):
With regard to physical, economic, psychological and social conditions, responsible parenthood is exercised by those who prudently and generously decide to have more children, and by those who, for serious reasons and with due respect to moral precepts, decide not to have additional children for either a certain or an indefinite period of time (HV 10; source).
The more recently promulgated Catechism of the Catholic Church uses the even lower standard of just reason:
A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. Moreover, they should conform their behavior to the objective criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life, the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart (CCC 2368).
In short, just reason suffices and just reason can include a desire to space children in such a manner that the parents' financial, physical, and emotional resources are not overtaxed. Those who maintain that only life-or-death reasons suffice to use NFP are obliged to demonstrate from Church documentation where the Church has required that.
By the way, it is true that you should refrain from mentioning your own opinion of your brother- and sister-in-law's family situation to them, especially because it is impossible to judge another couple's family from the outside. Providing relevant documentation of your position and requesting substantiation from them of their opinion will suffice.