Is NFP sinful?

As taken from Mari Immaculate Queen...http://www.cmri.org/03-nfp.htm :
"The very concept of “rhythm” was first considered by the Catholic Church in 1853. The Bishop of Amiens, France, submitted the following question to the Sacred Penitentiary:
“Certain married couples, relying on the opinion of learned physicians, are convinced that there are several days each month in which conception cannot occur. Are those who do not use the marriage right except on such days to be disturbed, especially if they have legitimate reasons for abstaining from the conjugal act?”
On March 2, 1853, the Sacred Penitentiary (during the reign of Pope Pius IX) answered as follows:
“Those spoken of in the request are not to be disturbed, providing that they do nothing to impede conception.”
* Please note: “providing that they do nothing to impede conception.” When married couples practice rhythm, they do not do anything unnatural in the act itself.

"For those who would belittle Pope Pius XII’s teaching on the morality of rhythm on the score that he addressed only mid-wives and nurses, let them realize that this address is contained in the Acta Apostolicae Sedis (the official Acts of the Apostolic See). Refer to:Acta Apostolicae Sedis 43 (1951) 845-46. On two other occasions, Pope Pius XII reiterated this same teaching and these also can be found in the Acta Apostolicae Sedis43 (1953) 855-60 at 859 and Acta Apostolicae Sedis 50 (1958) 732-48, at 736.
“God has endowed the nature of woman with both periods. Deliberately to limit the use of marital relations exclusively to the sterile periods in order to avoid conception (i.e., to practice periodic continence or rhythm) is, according to the common teaching of theologians, morally lawful in actual practice if there is mutual consent, sufficient reason and due safeguards against attendant dangers. “It is also common teaching that this practice of family limitation without good and sufficient reason involves a degree of moral fault. This fault certainly could be mortal if serious injustice is done or there exists grave danger of incontinence, divorce, serious family discord etc...."
"Well before Vatican II, moral theologians consistently reiterated the teaching of the Sacred Penitentiary and Pope Pius XII on the morality of rhythm. It is difficult to comprehend how anyone can claim that the pope, the Sacred Penitentiary, and moral theologians have been in error on this issue for some 150 years and that laity have now figured it out."
sedevacantist websites misinterprets Pope Pius XI in his encyclical Casti Connubii when he teaches:
“Since, moreover, the conjugal act by its very nature is destined for the generating of offspring, those who in the exercise of it deliberately deprive it of its natural force and power, act contrary to nature, and do something that is shameful and intrinsically bad.”
" Married couples do not “deprive it [the marriage act] of its natural force and power” with the practice of rhythm because conception is still possible."
" As regards the conjugal act spouses are free to choose whatever time they wish to use their marital rights or also to abstain by mutual consent. Thus they are not obliged to perform this act only during the fertile period, neither are they obliged to refrain during the sterile period."
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From Wikipedia: " St. Augustine. In the year 388, he wrote, "Is it not you who used to counsel us to observe as much as possible the time when a woman, after her purification, is most likely to conceive, and to abstain from cohabitation at that time"

Hence it is clear that Pius XI's solemn censure cannot be referring to NFP (periodic continence). He must be referring only to those conjugal acts which, if it were not for the unnatural intervention of one or both spouses, would have retained the said "natural power to generate life". In other words, the Pope's condemnation applies exclusively to conjugal acts carried out during what the spouses understand to be the wife's fertile period, but which they deliberately pervert (whether by 'withdrawal', condoms, pills, or any other technique) so as to deprive them of that fertility. They thus dare to raise their hands, as it were, against the approach of the Creator Himself; as if they were traffic policemen with the right to signal orders to the Lord, obliging Him to take a detour: "Stop! Halt! Go back! Not now! No entry allowed here for you!" Couples using NFP, on the other hand, are not guilty of any such presumption. They are respecting God's sovereignty over human life and death, and are simply following their God-given instincts, and using their God-given conjugal right, at those times when the Creator Himself has already made it clear, by the way He has fashioned human female biology, that He has no will to use their spousal love in order to create new life.