I don't really empathize with the push or the desire to limit the number of children. This is probably because I come from and have always lived in quite prosperous areas where numerous children are encouraged.
Also, in Pope Pius XII's 1951 address to midwives regarding the possibility of a fruitful apostolate, he paints a very daunting picture at the moral considerations necessary before jumping into what we call NFP:
'Today, besides, another grave problem has arisen, namely, if and how
far the obligation of being ready for the service of maternity is
reconcilable with the ever more general recourse to the periods of
natural sterility the so-called "agenesic" periods in woman, which seems
a clear expression of a will contrary to that precept.
'It is necessary first of all to consider two hypotheses. If the
application of that theory implies that husband and wife may use their
matrimonial right even during the days of natural sterility no objection
can be made. In this case they do not hinder or jeopardize in any way
the consummation of the natural act and its ulterior natural
consequences. It is exactly in this that the application of the theory,
of which We are speaking, differs essentially from the abuse already
mentioned, which consists in the perversion of the act itself. If,
instead, husband and wife go further, that is, limiting the conjugal act
exclusively to those periods, then their conduct must be examined more
'Here again we are faced with two hypotheses. If, one of the parties
contracted marriage with the intention of limiting the matrimonial right
itself to the periods of sterility, and not only its use, in such a
manner that during the other days the other party would not even have
the right to ask for the debt, than this would imply an essential defect
in the marriage consent, which would result in the marriage being
invalid, because the right deriving from the marriage contract is a
permanent, uninterrupted and continuous right of husband and wife with
respect to each other.
'However if the limitation of the act to the periods of natural
sterility does not refer to the right itself but only to the use of the
right, the validity of the marriage does not come up for discussion.
Nonetheless, the moral lawfulness of such conduct of husband and wife
should be affirmed or denied according as their intention to observe
constantly those periods is or is not based on sufficiently morally sure
motives. The mere fact that husband and wife do not offend the nature of
the act and are even ready to accept and bring up the child, who,
notwithstanding their precautions, might be born, would not be itself
sufficient to guarantee the rectitude of their intention and the
unobjectionable morality of their motives.
'The reason is that marriage obliges the partners to a state of life,
which even as it confers certain rights so it also imposes the
accomplishment of a positive work concerning the state itself. In such a
case, the general principle may be applied that a positive action may be
omitted if grave motives, independent of the good will of those who are
obliged to perform it, show that its performance is inopportune, or
prove that it may not be claimed with equal right by the petitionerin
this case, mankind.
'The matrimonial contract, which confers on the married couple the
right to satisfy the inclination of nature, constitutes them in a state
of life, namely, the matrimonial state. Now, on married couples, who
make use of the specific act of their state, nature and the Creator
impose the function of providing for the preservation of mankind. This
is the characteristic service which gives rise to the peculiar value of
their state, the bonum prolis. The individual and society, the
people and the State, the Church itself, depend for their existence, in
the order established by God, on fruitful marriages. Therefore, to
embrace the matrimonial state, to use continually the faculty proper to
such a state and lawful only therein, and, at the same time, to avoid
its primary duty without a grave reason, would be a sin against the very
nature of married life.
'Serious motives, such as those which not rarely arise from medical,
eugenic, economic and social so-called "indications," may exempt husband
and wife from the obligatory, positive debt for a long period or even
for the entire period of matrimonial life. From this it follows that the
observance of the natural sterile periods may be lawful, from the moral
viewpoint: and it is lawful in the conditions mentioned. If, however,
according to a reasonable and equitable judgment, there are no such
grave reasons either personal or deriving from exterior circumstances,
the will to avoid the fecundity of their union, while continuing to
satisfy to the full their sensuality, can only be the result of a false
appreciation of life and of motives foreign to sound ethical principles.'
'Now, the truth is that matrimony, as an institution of nature, in
virtue of the Creator's will, has not as a primary and intimate end the
personal perfection of the married couple but the procreation and
upbringing of a new life. The other ends, inasmuch as they are intended
by nature, are not equally primary, much less superior to the primary
end, but are essentially subordinated to it. This is true of every
marriage, even if no offspring result, just as of every eye it can be
said that it is destined and formed to see, even if, in abnormal cases
arising from special internal or external conditions, it will never be
possible to achieve visual perception.
'It was precisely to end the uncertainties and deviations which
threatened to diffuse errors regarding the scale of values of the
purposes of matrimony and of their reciprocal relations, that a few
years ago (March 10, 1944), We Ourselves drew up a declaration on the
order of those ends, pointing out what the very internal structure of
the natural disposition reveals. We showed what has been handed down by
Christian tradition, what the Supreme Pontiffs have repeatedly taught,
and what was then in due measure promulgated by the Code of Canon Law.
Not long afterwards, to correct opposing opinions, the Holy See, by a
public decree, proclaimed that it could not admit the opinion of some
recent authors who denied that the primary end of marriage is the
procreation and education of the offspring, or teach that the secondary
ends are not essentially subordinated to the primary end, but are on an
equal footing and independent of it.'
With this in mind, I think one has to be really very, very careful before jumping into NFP.