I'm sorry if I'm a bit defensive about this subject. Even taking the approach that we do --which is that the hard hitting catechesis that NFP is only to be used for grave reasons and that many Catholics do not have grave reasons-- takes up more time than teaching the mechanism of NFP itself...
I still have reservations that after teaching these truths some couples will use it selfishly. And I do not deny that there are some who teach NFP in the manner you describe. It can be done right, but it certainly is not always done right.