I beg your pardon! 2000 years of Catholic teaching says I am right. I'm not even sure where Humanae Vitae says I am wrong. Please show me. I know Paul VI spoke against contraception - but I am pretty sure (not positive) that he did not speak in favor of NFP. He conducted an extensive research into it, but did not endorse it
As far as a long list of Popes in favor of NFP - there is no such animal - Polycarp was lying, and broadsword is mistaken - badly.
I spent the morning surfing NFP sites, and not once did I find the phrase "use only inmatters of grave importance" or whatever the phrase is.
NFP is a lifestyle promoted to navigate around Catholic teaching, and falls far short to the discerning, but fills the needs of the gullible. I would be willing to bet that it is promoted by those wanting to tear down the Church brick by brick.
I know there is a problem with having too many babies, I'm a father of four myself. And I am not saying the are not legitimate problems. But strictly speaking, one is not trusting in the Lord if they seek to limit their offspring. Leave that to the pagans.
On the practical side, let your conscience be your guide, but don't be guided by those, even Priests, who counsel different than what the Church teaches. NFP is just another form of contraception, and the organization is no different than planned parenthood.
Jesus will watch over His Catholic babies.