When NFP is used by people who take it semi-seriously they have 7.5% chance of getting pregnant per cycle, to put that in perspective couples having frequent sex with no contraception of any type have 28% chance of pregnancy per cycle.
Because differences like this, though generally not this large, are common, when measuring birth control effectiveness: means are evaluated using a "typical use" scale not a "perfect use scale". So for example condoms when used every time and with a spermicidal gel have a 98% effectiveness rate (i.e. with perfect use a sexually active woman will get pregnant only 2% of the time). When used by actual people mistakes happen and the actual actual effectiveness rate is measured at 85%. Where NFP methods are heavily used unintentional pregnancy rates among sexually active women are about 24%. Considering we are talking several decades of sexually active fertility even a 10% failure rate would mean 2-3 extra children over the course of a woman's lifetime.
Western women seem to be heading towards a fertility rate of 1.3 children per woman, and even in America non immigrant woman are at 2.1 children per woman; NFP simple doesn't seem effective enough in the absence of heavy use of abortion.
Moreover, there is a bit of irony. If one adopts the currently fashionable definition that life begins at conception NFP greatly increases the incidence of implantation failure, which would be miscarriage under this definition, by encouraging sexual activity during the period when women still conceive but the fetus tends to fail to implant successfully. NFP, the Catholic church's recommended practice causes vast vast numbers of natural abortions over the course of a couple's life. So I would strongly disagree with the church, that it is not the intentional killing of children, if one defines life to begin at conception, and one defines "intent" in any consistent way.
The church is simply aiming for an irreconcilable situation:
- They have over the last 200 years redefined abortion to apply much earlier than quickening, i.e. when the woman first feels fetal movement. This eliminates the sorts of birth control methods that were popular in previous centuries, which we would today call "abortion inducing drugs". It also introduces the moral issues with NFP I cited above.
- They have redefined marriage to be primary about sex rather than primary about property and legitimate heirs. Thus there is no longer any distinction made between non-marital and marital pregnancy, as well as making much distinction between adultery and fornication. This to some extent is compounded in our society that has moved towards late marriage.
- They dismiss artificial contraception of virtually any type as immoral. Thus eliminating the only means humans have discovered that in a widespread and reliable way is capable of keeping a woman's fertility down to 1-3 children per lifetime without heavy use of abortion (in the modern sense of the word).
- They do aim for their standards to be adopted in a widespread way, and not seen as just theoretical goals that no one in practice actually follows.
If the church wants Humane Vitae to be taken seriously they either need to indicate:
i) What is the unknown secret for massive energy production to allow for a growing population?
ii) How to maintain fertility at the rate of around 1-2.25 children per woman over the course of their life in practice using NFP?