There are women for whom the Creighton Napro or Billings methods work 100%.
Blessed are they. Thermometers are a waste of time as they measure the increase in temp after ovulation has already started. Blessed are those women who have such regular unchanging cycles that they can predict their ovulation. Even more blessed are those women who have some other indicator like pains or breast tenderness or vulva swelling that they recognise. However for many women no natural family planning method will work due most possibly to the stresses and strains of family life. Until the Catholic Church starts funding research into providing a fool proof method of testing wether the mucus in her cervix will carry sperm into the ovaries via the Fallopian tube, perhaps a computer that uses a pin prick of blood or a sample of vaginal mucus for its analysis and it’s cheap like a telephone, women will never be free to space their family properly and or to love their existing children adequately. Ultimately most Catholics use NFP exactly like condoms, they have 2 or 3 kids have no intention of having anymore, but know that the method works perfectly for them and so they pretend that they are putting off children indefinitely. For the Catholic Church to say condoms are not equivalent of NFP they would have to bring in another rule for peoples sex lives. One that says if a catholic woman is using NFP then she must have one last child just before menopause begins.
How is determining ovulation via thermometer a “waste of time”? In strict NFP, determining ovulation is exactly what you want to do! The thermometer is $3-$5. Determine that ovulation occurred via prolonged temperature rise of 4 or, if you want to be really, really safe, 5 days, and voila! There is no way to get pregnant that cycle. An egg lives 24 hours post-ovulation, and double ovulation occurs within 24 hours of the first. Most women will then have a minimum of a week, and in most cases more like 10 days, before their period begins.
That people might be hypocritical about a belief doesn’t mean the belief isn’t true.
As others have pointed out, all methods of avoiding conception, excepting total abstinence, fail sometimes. This is why some couples in bad situations choose total abstinence. I personally know of one such couple who were in a situation wherein the wife would die if she got pregnant again, so they chose to abstain until she was in menopause.
No method (except, as I mentioned, abstinence) will work perfectly for all women. That doesn’t mean that methods which are wrong ought to be used in the place of ones which are right.
ETA passage from Humanae Vitae, the encyclical on Church teaching in this matter.
"16. Now as We noted earlier (no. 3), some people today raise the objection against this particular doctrine of the Church concerning the moral laws governing marriage, that human intelligence has both the right and responsibility to control those forces of irrational nature which come within its ambit and to direct them toward ends beneficial to man. Others ask on the same point whether it is not reasonable in so many cases to use artificial birth control if by so doing the harmony and peace of a family are better served and more suitable conditions are provided for the education of children already born. To this question We must give a clear reply. The Church is the first to praise and commend the application of human intelligence to an activity in which a rational creature such as man is so closely associated with his Creator. But she affirms that this must be done within the limits of the order of reality established by God.
**If therefore there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way which does not in the least offend the moral principles which We have just explained. **(20)
Neither the Church nor her doctrine is inconsistent when she considers it lawful for married people to take advantage of the infertile period but condemns as always unlawful the use of means which directly prevent conception, even when the reasons given for the later practice may appear to be upright and serious. In reality, these two cases are completely different. In the former the married couple rightly use a faculty provided them by nature. In the later they obstruct the natural development of the generative process. It cannot be denied that in each case the married couple, for acceptable reasons, are both perfectly clear in their intention to avoid children and wish to make sure that none will result. But it is equally true that it is exclusively in the former case that
husband and wife are ready to abstain from intercourse during the fertile period as often as for reasonable motives the birth of another child is not desirable. And when the infertile period recurs, they use their married intimacy to express their mutual love and safeguard their fidelity toward one another. In doing this they certainly give proof of a true and authentic love."
In the highly unlikely case you describe, the above bolded passages would apply to the question of whether the couple can have sex if they think that it’s very likely they shouldn’t have any more children.
Yes to all of this. I’d also recommend some light manipulation. Your bedroom should be completely dark - heavy black out shades, cover the alarm clock, don’t recharge your phone in your bedroom. Zero light. Then, when your fertility goes high and you start to have some slippery mucus you leave a very low watt nightlight on in the bedroom. It can spur ovulation. If you don’t want to do that manipulation, just sleep in complete darkness. Light, particularly blue based lights can wreak havoc with your cycle. If you need to turn a light on in the middle of the night, put a red bulb in a lamp and use that.