Thus if the intent, the "end" is to avoid pregnancy, it is really splitting hairs to say it is okay in one case, and a terrible sin in another case.
This is what appears to be the precise locus of moral confusion here.
There is no sin of "contraception". Provided that duties towards social justice are being met, there is no sin for a married couple to not have additional children.
Sin is involved in artificial birth control because it frustrates the natural workings of an action and is meant to satisfy lustful intent.
The sin of artificial birth control is primarily and always the use of unnatural means of sexual intercourse and secondarily only possibly a sin against social justice of the end of not having children.
Of course NFP does not make sense if you invent a sin of being married and not having children.
Exactly. What do the condemnations of contraception by the Church say?
"Similarly excluded is any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means." Paul VI, Humanae Vitae, 14
"Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious." Pius XI, Casti Connubii, 54
Nothing about "and they won't end up having enough children" mentioned there.
Unfortunately confusion has been created by mixing the duty, in justice, of having children - which is imposed upon all married couples which make use of their marriage, unless just causes exist - with the prohibition of sins against the nature of the conjugal act. Unless these are properly distinguished, the confusion evident in this thread will result.
Thank you for this admission. We agree that "NFP does not make sense" as long as marriage imposes an obligation to have children. You yourself have often stated that the "sin of being married and not having children" is not an "invention," but a reality, although you qualify that statement by saying that "4 is enough."
Here is what Pope Pius XII stated on the topic:
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 petitioner—in this case, mankind.Therefore, following your own reasoning, "NFP does not make sense," since all couples who make use of their right to conjugal intercourse have imposed upon them a positive obligation to "fruitful marriage."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.
Artificial contraception is an illegitimate means in and of itself, but even licit means become "a sin against the very nature of married life" if they are used to frustrate the primary purpose of marriage and to avoid one's positive obligation to procreation.