The reason NFP must only be used when just reasons exist
The new documents tend to fall on the use of the term* serious reason*. Hence I prefer this term.
Here are some examples of more recent documents:
From the Encyclical of Pope John Paul II Evangelium Vitae
This happens when the family is generously open to new lives, and when couples maintain an attitude of openness and service to life, even if, for
serious reasons and in respect for the moral law, they choose to avoid a new birth for the time being or indefinitely.
From the Compendium of the Catechism of the Catholic Church – approved by Pope Benedict XVI
- When is it moral to regulate births?
2368-2369
2399
The regulation of births, which is an aspect of responsible fatherhood and motherhood, is objectively morally acceptable when it is pursued by the spouses without external pressure; when it is practiced not out of selfishness but for
serious reasons; and with methods that conform to the objective criteria of morality, that is, periodic continence and use of the infertile periods.
From the Compendium of Social Doctrine of the Catholic Church
- The family contributes to the social good in an eminent fashion through responsible motherhood and fatherhood, the spouses’ special participation in God’s work of creation[519]. The weight of this responsibility must not be used as a justification for being selfishly closed but must guide the decisions of the spouses in a generous acceptance of life. “In relation to physical, economic, psychological and social conditions, responsible parenthood is exercised both in the duly pondered and generous decision to have a large family, and in the decision, made for serious reasons and in respect of the moral law, to avoid for a time or even indeterminately a new birth”[520]. The motivations that should guide the couple in exercising responsible motherhood and fatherhood originate in the full recognition of their duties towards God, towards themselves, towards the family and towards society in a proper hierarchy of values.
From a vatican guide for confessors:
The witness of couples who for years have lived in harmony with the plan of the Creator, and who, for
proportionately serious reasons, licitly use the methods rightly called “natural,” confirms that it is possible for spouses to live the demands of chastity and of married life with common accord and full self-giving.
(Noting again that serious need not = “life or death serious”)