The theological basis for Natural Family Planning (NFP) was established by the Council of Trent.
The Council of Trent: “If anyone says that the Church errs when She decrees that, for many causes, a separation may take place between the spouses, in regard of bed or in regard of cohabitation, for a determinate or indeterminate period: let him be anathema.” [Canon 8, On Matrimony]
It is moral for spouses to choose, by mutual agreement, “a separation … in regard of bed”; in other words, they may refrain from marital relations. And this separation may take place “for a determinate or indeterminate period” of time. What reasons permits spouses to make this choice? They may do so “for many causes”, that is to say, for many reasons, in various circumstances.
This teaching establishes that NFP is not intrinsically evil. The foundation of NFP is to choose to refrain from marital relations for a determinate period of time. So if anyone claims that NFP is intrinsically evil, they contradict this infallible teaching of the Council. Catholic spouses are free to use NFP; it is not necessarily sinful.
And this also implies that NFP has a good moral object. Every human act has a moral object (or objects), and each object is either good or evil. Some acts, with their objects, are neither virtuous, nor sinful; but these acts are morally permissible. So the object is at least a natural good.
The “many causes” assertion also has a particular bearing on NFP. Some Catholics say that NFP is only moral for a grave reason. However, as an act which has a good moral object, the usual analysis of the three fonts of morality applies. The spouses must have only good in their intention: to space out the births of their children; to avoid procreating more children than they can support; to avoid placing a harmful burden on the health of the wife (if she is ill), etc.
As for the circumstances of the act, as long as the reasonably anticipated good consequences morally outweigh (or at least equal) the reasonably anticipated bad consequences, the use of NFP would be moral. Refraining from marital relations for a definite or indefinite period can be moral “for many causes”, and this implies that said refraining does not necessarily have gravely harmful consequences.
Therefore, the couple may use NFP for any of a number of good reasons. They do not need a grave reason unless they plan on using NFP for a long time, with the objective of avoiding all (further) procreation of children. Example of grave reasons include if the wife’s life would be threatened by a subsequent pregnancy, or if the conceived child would have a severe birth defect.
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