Everyone agrees that if spouses were using ABC the same way they would use NFP, it would be wrong. NFP therefore is not intrinsically wrong, they say, otherwise it would always be a sin to have relations when the wife is infertile.
This is a straw man. My contention is that NFP is intrinsically wrong. It's all about the intention. Spouses who use the fertile and infertile days together may be using the infertile days like an NFP user, yes, but they do not intend to contracept. This is the difference.
It would be extremely hard to say that withdrawal is somehow "unnatural" while NFP is "natural" anyway. Who was the Merriam or the Webster that decided on the definition of "natural" as an act where the sperm is deposited where it is supposed to be deposited? I could just as easily define natural as "the absence of body-altering substances or chemicals." That means withdrawal is also natural -- does that mean it's okay?
Not only that, but many "unnatural" means are approved by the Church for good purposes, such as taking drugs to heal illnesses. If there were ever a good reason to have sex while premeditating how not to have children, the Church could allow artificial birth control for that too. But it doesn't.
The NFP defenders say that is because artificial birth control is intrinsically evil. But when forced to explain WHY artificial birth control is intrinsically evil and why NFP isn't, they resort to feeble excuses that artificial birth control prevents the man from giving himself entirely to the woman, or that he withholds part of himself when he uses a condom, or even that the condom is not respectful to the woman. I can't remember any such soppy sentimentality about sex in the entirety of Church history, I can't think of a single saint talking about the "giving of oneself intimately" and so on. It all reeks of the New Romantic pseudo-theology of Dietrich von Hildebrand, you know, where the shackles of clinical Catholic sex are thrown off and sex becomes a sacramental communion, where love between the spouses is placed above duty, where buzzwords like "giving," "sharing," and "togetherness" are the order of the day. This led to the error of Vatican II where the primary and secondary ends of marriage were reversed in all but name.
Many, many new Catholics or Protestants investigating Catholicism, if you read about NFP, are smacked upside the head by this, and they call it what it is -- hypocrisy. Those who believe in NFP can never really defend it without getting tangled up; so eventually the argument always comes down to the same thing: Blind faith.
It is good to have blind faith in the Church but not when the Church has been taken over. Blind faith in Pius XII, who I've shown in many ways speaking in a double-minded, communistic way, and a bunch of theologians who later became leading lights of Vatican II? No thanks.