So...saying that homosexuality is normal would not be heresy for a Catholic? Or saying adultery is not sinful would not be heresy for a Catholic?
Or...saying that CONTRACEPTION is morally licit is not heresy?
Remember, the deposit of Faith includes doctrines and dogmas concerning Faith and Morals. We are NOT discussing a mere matter of Church discipline here.
So, one may commit heresy in matters of Morals, if the opinion is contrary to a defined teaching of the Ordinary Magisterium.
Therefore, one may indeed commit heresy by maintaining that NFP is always sinful.
No, this is wrong, and Fr. Harrison never makes any such claim. He merely denies the counter claim that it is a heresy to approve of NFP. He never says that it is a heresy to claim that NFP is sinful.
First of all, NFP has never been defined as Church doctrine. As Fr. Harrison correctly states in his article, Pope Pius XI used language that seemed to invoke the power of infallibility in "Casti Connubii" when he stated that any action which frustrates the marriage act in its natural power and purpose is gravely sinful. Some claim that this can be applied to NFP, at least in some circumstances. It would certainly not be a heresy to say so.
On the other hand, Pope Pius XII approved of NFP in "Allocution to the Italian Midwives." But it would fall far short of heresy to claim that he was mistaken. Fr. Harrison tries to create a claim of long-standing tradition for his position by citing the pronouncements of the Sacred Penitentiary. This claim may or may not stand up to closer scrutiny. But we are certainly a long way from moral unanimity.
One may commit heresy in a matter of morals--but the NFP is a relatively new debate within the Church. It is not so clearly established as the other issues you mention. We are talking about the deposit of faith, after all, which derives from what we've been bequeathed from the apostles. All of which would suggest a certain antiquity of opinion at least.
So what you are saying there is that NFP is not always sinful, but is mostly sinful.
Spoken like a true deceiver. At the same time that you are luring people into using NFP, you subtly slip in to the conversation that it is mostly sinful. Let's see, who else does that? Hmmm?