Can NFP be a mortal sin if a couple is open to life, but is delaying pregnancy for less than grave reasons?
NFP is morally neutral. It is a technique, intended to track fertility, and can be used either to avoid or to achieve pregnancy. At issue here is not NFP but the couple’s decision to postpone having children.
I cannot comment one way or another on whether or not a couple’s reasons are valid. All I can say is that the Church does not require a couple to have “grave” reasons for avoiding pregnancy. In the Catechism of the Catholic Church, the Church teaches that what is required are “just” reasons:
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).