NFP, as it is usually taught and practiced today, is not authorized in
Humane Vitae. NFP is a bundle of scientifically-based techniques/technologies that help identify, through certain "actions," the wife's fertile time. The "actions" required by those NFP techniques and technologies are, by their nature, "artificial." On this point,
Humanae Vitae says,
"Similarly excluded is any action which either before, at the moment of, or after sɛҳuąƖ intercourse, is specifically intended to prevent procreation—whether as an end or as a means." (Section 14)So, if the couple is taking any "action," such as using thermometers and comparing the readings of the thermometers to a scientifically-determined fertility table, and the couple takes such actions with the specific intention to prevent procreation, then the couple has violated the law of the Church, according to
Humanae Vitae.
On the contrary, if the couple simply "winged-it," not taking any preparatory, preventative actions before having sex, but just avoided sex on what their common sense told them might be infertile days, then they would not sin, in "deed," because their "inaction" is not an "action," and so, would not be covered under the restriction quoted above.
But
Humanae Vitae (Section 16) also teaches that any desire ("thought") to have sex while at the same time wishing to avoid pregnancy requires that the couple have this desire for "well-grounded" reasons, and that their desire to avoid a pregnancy in that particular sɛҳuąƖ act is not a permanent desire, but rather only a temporary desire to "space births" because of some serious physical or psychological reason which is expected to pass.
So, in a roundabout and somewhat vague way,
Humanae Vitae does uphold the traditional teaching.
Finally, to be fair, the techniques taught by the NFP books could, theoretically, be used to help determine the fertile window and be used by the couple with
the intention to increase the chances of procreation (if they were having infertility issues). In that case, using NFP techniques would not be unlawful because the couple using NFP, in that case, do not intend to prevent procreation.