—St. Augustine (388AD), De moribus Ecclesiae catholicae et de moribus Manichaeorum II, 18

Sounds to me like some "conservative Novus Ordo" people who erroneously conclude that you can use NFP for any reason, or no reason at all, and it's okay, because after all, it's NFP, and you're making some kind of sacrifice to be able to use it in the first place, so no problemo, end of story.  There are people who really think like that.  No joke.  I've had people say that to me.

This is in direct opposition to the teachings of Pius XI, Pius XII, and Paul VI in Humanae vitae.  I do believe St Jean Vianney had something to say about the women who go to hell because they did not give God the children they should have.

I thought the knowledge of how to do that was not discovered until modern times?

Whatever anyone says the intention of NFP is to avoid conception.  It is birth control.  

To the extent that it is morally acceptable it is only a alternative to complete abstinence.  Where the abstinence would cause othe sin (divorce, adultery or masturbation).  

That's simply not true. If the obligation to the common good is met, there is no issue.

Read the comments of gbcdoj:
https://freerepublic.com/focus/f-religion/1165258/posts

http://www.traditionalmass.org/articles/article.php?id=6&catname=9

On the Question of Natural Family Planning – CMRI: Congregation of Mary Immaculate Queen https://cmri.org/articles-on-the-traditional-catholic-faith/on-the-question-of-natural-family-planning/

If the obligation to the common good is met,

Common good?  You’re reducing morality to social measurements? What a joke!  NFP, unless used to conceive, or for VERY grave reasons, is against the natural law.  It’s a sin against the sacrament of matrimony itself. 

I didn't "reduce" anything, I'm showing what a moral theology manual said from prior to V2, if you bothered even reading the links I posted. I provided numerous articles that show that isn't the case. That was just one example.

I never said it was ok to use all times and everywhere and in every case. Distinctions matter. Chill out.

People jump to conclusions way to fast on this forum.

The Church has never forbidden Catholic couples from spacing children say if they have a large-ish family already (4+ in 6 years for instance). Humanae Vitae makes for provisions for this, as does other moral theology manuals.

We can't just blanket say it's sinful in every case, that's simply not true, and trads need to stop saying it, they just assume everyone is using it for selfish intentions. If a mother is about to blow her brains out cause she is homeschooling a lot of small children, plus taking care of the home, with basically no help from the outside world, these are psychological considerations which can be taken into account for instance, Humanae Vitae basically says this. For instance I know of at least 2 mothers who've had mental breakdowns who were considered trads, devout, with large families.

I just think it's very dangerous to automatically tell people it's a grave sin against matrimony when we don't know people's situations.

^^^This... while respecting all those heroic Catholic couples who have all of the children God sends them, without so much as thinking about whether the wife is fertile or not.  That is perfect abandonment to God's Will.  Breastfeeding already naturally spaces children, in most cases, about two years, regardless of any fertility awareness the couple might bother to observe or ignore.

The summary provided by Bishop Pivarunas was excellent and I have bookmarked it.  Perhaps I can share this with some of these conservative Novus Ordo wacked-out Vatican II "holy rollers" who vow and declare "anytime you use NFP is okay because it's NFP, requires sacrifice, and isn't artificial or onanistic".  They're just ignorant.  They are the same types who dogmatically crow that "the soul enters the body immediate at conception" (as it well may, but we cannot prove that) because the Catechism --- that mammoth-sized "one big encyclical on everything" that these people spend hours and hours reading and re-reading (ever heard of the catechisms of Trent and St Pius X, eh?) --- uses the word "immediately" in the English translation.   They're not intellectually sophisticated enough to comprehend that, in this context, "immediately" means "directly created by Almighty God without any mediation or agency by the parents".  Granted, that's not the everyday use of the word "immediately", but this is yet one more example of people knowing just enough about the Faith to be a spiritual danger both to themselves and to others.  They would likely just say "he's a sedevacantist", which in their lexicon would mean that everything he says is wrong.  I'm not a sedevacantist either ("sede-agnostic" would be more like it) but I love and seek truth wherever I might happen to stumble across it.