Using NFP to avoid having children for no other reason than just not wanting them is also sinful.
I agree with you, but some faithful Catholics of good will maintain that NFP may be used absolutely at the discretion of the couple, reasons notwithstanding, or for no reason at all other than just, as you say, not wanting to have children.
I am considering just how to go about making an inquiry to the Holy See (probably Sacred Congregation for the Doctrine of the Faith) as to whether this is licit, and whether, at some point within the marriage, the couple
at the very least must disregard the natural cycles (more below) for a period of time long enough to be open to the conception of a child without intending otherwise. My
sensus catholicus or “Catholic spidey sense” tells me that this is too nuanced of a question, and that there are just too many variables, for the Holy See to be able to say “yes,
this reason is valid, but
that reason is not valid”. Nonetheless, past Popes (Pius XI, Pius XII) have said that for “grave” or “serious” reasons, NFP may be used, and then the question that begs to be asked is “but what about reasons that are
not grave, or are
not serious, what then?”. And would it be a sin to use NFP selfishly, where the sin would be in the selfishness, not in the use of NFP
per se? Or would we say “if you’re using NFP, that’s selflessness enough, your
bona fides is assumed”?
Some could object, “well, then, this means that the Church is forcing married people to have relations”. Not true. Any married couple can always mutually resolve not to have relations, either for a period of time, or permanently as in “Josephite marriages”. But if they
do choose to have a normal, healthy conjugal life, the question then is “do they disregard the naturally occurring indicators of female fertility, or do they regard them and act accordingly, either to engage in them if they want to try to conceive a child, or not to engage in them if they wish to avoid conceiving?”. If they regard these indicators and are seeking not to fall pregnant, presto, they’re using NFP! Simple as that.