It comes from standard Catholic handbooks of Moral Theology. Example:
MORAL THEOLOGY: A Complete Course * Based on St. Thomas Aquinas and the Best Modern Authorities * By JOHN A. McHUGH, O.P. And CHARLES J. CALLAN, O.P. REVISED AND ENLARGED BY EDWARD P. FARRELL, O.P., Vol. 2:PART II SPECIAL MORAL THEOLOGY (Continued)
THE DUTIES OF MEMBERS OF SOCIETY
Art. 2: THE DUTIES OF MEMBERS of DOMESTIC AND CIVIL SOCIETY ...2622. Is Birth-Control Ever Lawful? -- (a) If this refers to an * end * (viz., the limitation of the number of children or the spacing of their arrival), it is not unlawful in itself (see 2617) ; and it is sometimes a duty, as when the wife is in very poor health or the family is unable to take care of more.
But in view of the decline and deterioration in populations today, it seems that couples who are able to bring up children well should consider it a duty to the common welfare to have at least four children, and it should be easy for many to have at least a dozen children. The example of those married persons of means who are unable to have a number of children of their own, but who adopt or raise orphaned little ones, is very commendable.
(b) If birth control refers to a * means * of family limitation, it is lawful when that means is continence or abstinence from marital relations, not if it is onanism or the use of mechanical or chemical means to prevent conception. The objection that husbands cannot restrain themselves is really an insult to God's grace and is contradicted by numerous facts. A man of manly character should be ashamed to admit that he is the slave of passion, and the fact that God commands chastity and that millions obey Him both in the wedded and single state is sufficient proof that, even though hard, sexual abstinence is not impossible, if there is a real resolve and the right means are employed, such as rooming apart and concentration on other and higher things.
Continence or abstinence is counselled by the Church should conditions make the conception of children inadvisable. It is counselled, not commanded, since it involves heroic sacrifice which makes it all the more meritorious and praiseworthy: "It is wronging men and women of our times to deem them incapable of continuous heroism. Today, for many reasons -- perhaps with the goad of hard necessity and even sometimes in the service of injustice -- heroism is exercised to a degree and to an extent which would have been thought impossible in days gone by. Why then, should this heroism, if the circumstances really demand it, stop at the borders established by the passions and inclinations of nature? The answer is clear. The man who does not want to dominate himself is incapable of so doing. He who believes he can do so, counting merely on his own strength without seeking sincerely and perseveringly help from God, will remain miserably disillusioned" (Pope Pius XII, * Allocution to the Italian Catholic Union of Midwives *, Oct. 29, 1951).
Another lawful means of family limitation is "periodic continence" or "rhythm," the deliberate avoidance of conception by restricting intercourse, temporarily or permanently, to the days of natural sterility on the part of the wife. Many of the faithful are under the impression that the system has received the unqualified approval of the Church, that it constitutes a form of "Catholic Birth-Control." This is not completely true.
All theologians agree that the use of marriage during the sterile period is not * per se * illicit. The act is performed in the natural way; nothing has been done positively to avoid conception; and the secondary ends of matrimony, mutual love and the quieting of temptation, have been fostered. "If the carrying out of this theory means nothing more than that the couple, can make use of their matrimonial rights on the days of natural sterility, too, there is nothing against it, for by so doing they neither hinder nor injure in any way the consummation of the natural act and its further natural consequences" (Pope Pius XII, ibid).
"If, however, there is further question -- that is, of permitting the conjugal act on those days exclusively -- then the conduct of the married couple must be examined more closely" (ibid).
The following points summarize papal teaching on this aspect:
1) A premarital agreement to restrict the marital * right * and not merely the * use * to sterile periods, implies an essential defect in matrimonial consent and renders the marriage invalid. 2) The practice is not morally justified simply because the nature of the marital act is not violated and the couple are prepared to accept and rear children born despite their precautions. 3) Serious motives, (medical, eugenic, economic and social), must be present to justify this practice. When present, they can exempt for a long time, perhaps even for the duration of the marriage, from the positive obligations of the married state. 4) The married state imposes on those who perform the marital act the positive obligation of helping to conserve the human race. Accordingly, to make use of the marital act continuously and without serious reason to withdraw from its primary obligation would be a sin against the very meaning of conjugal life (Ibid).
Pope Pius explicitly confirmed the common teaching of theologians:
1) Rhythm, by mutual consent, for proportionate reasons, and with due safeguards against dangers would be licit. 2) Without a good reason, the practice would involve some degree of culpability. Not expressly confirmed, but simply an expression of common moral principles is the common agreement: 3) That the sin could be mortal by reason of injustice, grave danger of incontinence, serious family discord, etc.
Since the * Allocution *, the more common opinion in this country asserts that the Holy Father taught: 1) that married people who use their marital right have a duty to procreate; 2) that this duty is binding under pain of sin; 3) there are, however, reasons that excuse the couples from this obligation and, should they exist for the whole of married life, the obligation does not bind them at all; 4) the sin does not consist in the exercise of marital rights during the sterile periods; but in abstention from intercourse during the fertile periods precisely to avoid conception, when the couple could have and should have made its positive contribution to society. Sin is present when the practice is unjustifiedly undertaken; 5) the formal malice of illicit periodic continence is not against the sixth commandment; i.e., against the procreation of children or the use of the generative faculty, but against the seventh commandment, i.e., against social justice. The couple is not making its contribution to the common good of society; 6) from 4 and 5 above, it follows that the individual acts of intercourse during a period of unjust practice of rhythm do not constitute numerically distinct sins. Rather, granting the continuance of a single will act to practice rhythm, there is one sin for the whole period of illicit abstention during the fertile periods.
Since the Pope abstained from an explicit statement on the gravity of the sin, the controversy of whether the practice intrinsically is a mortal sin or not continued. The opinion in this country which holds the greatest authority states that mortal sin is involved in the ease of continued practice with a total exclusion of children and frequent use of marital rights during the sterile period.
Diversity of opinion has arisen as to the means of estimating when a serious sin has been committed. Some have used a temporal norm, e.g., unjustified use of rhythm for five or six years would constitute a serious matter. Undoubtedly most of the proponents of this norm would not accuse a couple of certain mortal sin if they already have one or more children; after that, indefinite use of the practice without excusing causes would not be a mortal sin. (This is admitted by most theologians.) Others have proposed a numerical norm as a basis to determine whether or not a couple has made its contribution to the conservation of the race. Concretely the proponents of this theory regard four or five children as sufficient to satisfy the obligation in such a way;
a) that the use of rhythm to limit the family to this number is licit provided the couple is willing and morally able to practice it;
b) that the limitation through rhythm to less than four requires a serious justifying cause. The intention involved to prevent conception would be seriously sinful in itself, since it causes great harm to the common good and involves in practice subordination of the primary to the secondary end or ends of matrimony. At the present time this opinion seems to be more favored in America than the first which places the gravity of the sin in the unjustified practice of rhythm for five years. (For a survey of recent opinion, see * The Conference Bulletin of the Archdiocese of New York *. Vol. XXXIV, No. 1, pp. 36 ff.)
On the other hand, some European theologians have denied that the practice constitutes a mortal sin in itself, independently of circumstances such as injustice and danger of incontinence.
The present state of opinion, then, is definitely undecided and calls for caution both in dealing too severely with penitents or too readily recommending the practice. The response of the Sacred Penitentiary of June 16, 1880, affords a safe guide in practice: "Married couples who use their marriage rights in the aforesaid manner are not to be disturbed, and the confessor may suggest the opinion in question, cautiously, however, to those married people whom he has tried in vain to dissuade from the detestable crime of onanism."
As to the theological censure to be attached to "rhythm," it is not approved, nor recommended, but seems to be tolerated for sufficiently grave reasons. * "Instead of being freely taught and commended, it is rather to be tolerated as an extreme remedy or means of preventing sin (Official Monitum *, Patrick Cardinal Hayes, Sept. 8, 1936, * Conference Bulletin of Archdiocese of New York *, Volume XIV, No. 2, p. 78).
Keep in mind that the above was written in the days of free Catholic schooling, cheap housing, and good industrial jobs for high school drop-outs. Today's circumstances make it somewhat less possible for "many to have at least a dozen children". The duty of having four children has not vanished, and cannot.
You either seem to be in the "don't ever need to have serious reasons for NFP" camp, or that you don't get to move into the non-serious reasons category until you've carried out your duty (to the best of your ability) to have 4. Which is it, or is it none of the above?
I'm not in either "camp". If you get to know me more, you'll soon find that my "opinion" such as it is, in matters religious, is the plain teachings of the Holy See. There is no surer guide than this. There are a few basic principals which need to be kept firmly in mind.
1) No married person is obliged to ever have sex except if their spouse requests it.
2) Entering the married state imposes certain duties to society upon the recipients of the sacrament, chief among which is the conservation of the human race through procreation.
3) Married couples can make use of the sexual rights as they see fit, providing they fulfill their duties and do not violate objective norms.
4) Having children is not only a duty of being married, but also a special blessing.
5) The Church recognizes that we must be governed by reason in our actions and we should not bring children into the world that we cannot care for. Our own opinion of this, regard being given to point (2) above, is a "serious reason".
Gathering these strands together, it can be shown that having a minimum of four children objectively provides for the continued growth of the human race and provides a minimum surplus of one per family for the Priesthood and Religious life. It can also be shown that the Church leaves us free to run our lives as we see fit, provided we fulfill our duties and avoid sin. Therefore, if a person has or realistically intends to have four children during their marriage (I say realistically, because it is not realistic for a 35 year old, for example, to assume they will have four children; so there is little justification for a 35 year old newlywedded women to use this practice), one may use NFP as often as one likes without any fault, provided this does not drive one's spouse to any acts of incontinence.
NFP is neither good or bad - it is a tolerated neutral. It can be used for either good or ill depending on intention. Those who are using NFP are not to be disturbed in their use of it, provided they are fulfilling the Christian duties of Marriage and are not sinning because of it.
The only reason one would need to consult a priest about using NFP is if one feels a need to use it in such a manner that ones duties as a married person would not be fulfilled. A concrete example are people who think that their having had one or two children is "plenty for me", therefore, I can use NFP.