NFP is actually not supposed to be used as a preventative tool. It is actually supposed to be used for a married couple to learn when is the best time to CONCEIVE. Obviously, it will be used for contracepting as well, but that is not its intent. It also brings a couple closer together as they both take part in planning their families.
Another point about about contraceptives. The pill is not a contraceptive, but an abortifacient. The IUD is also an abortifacient. Many contraceptives are really abortifacients in the guise of birth control.
Some pills don't reliably prevent ovulation, but do prevent implantation of the fertilized egg in the uterine wall. That is arguably abortifacient because the egg has already been fertilized. The "mini-pill" or very low dosage pill falls into this category . . . which is why such pills (e.g. Yaz) are marketed not as birth control but for regulation of dysmenorrhea and menorraghia (pain and excessive bleeding during menstruation).
The older pills, especially the heavy hitters like Ortho-Novum and Ovulen 21, absolutely prevent ovulation if taken properly. The multi-phase pills like TriPhasil are not an absolute guarantee because they have to be taken faithfully at the same time every day.
Of course, if people can't follow directions all bets are off.
This is a pet peeve of mine. NFP is in no way, shape or form contraception and can never be used as contraception. NFP does not act against conception, ever, whether it is used to postpone pregnancy or not. That's part of the beauty of NFP, it doesn't keep the body from acting naturally, because if you don't have sex, it is natural to not get pregnant.
Now, NFP could be properly referred to as birth control, because it does allow for both birth and self-control. But it ain't contraception.....