Their prayer lives are not the reason they are doing the NFP.
You seem to be missing the point in a big way.
Scripture tells us the only reason for abstinence in marriage is for a short time for the purpose of prayer.
It does NOT endorse it as a family planning method or for any other reason.
"You seem to be missing the point in a big way."
You, first of all, don't know whether their prayers lives are the reason they are doing the NFP. Their prayer life may be centrally involved in the process and the motivation--- and usually is, at least as far as I can see.
Second, the impulse to attribute non-prayerful, non-God-pleasing reasons for NFP, even among the most ardent of believers --- who are the very ones most likely to opt for NFP --- shows a most unattractive reflex to assume the worst of others, rather than be best.
Third, all the NFP literature I have seen (from the Christian, rather than the health/selfcare/medical sources) emphasizes the importance of prayer and utter reliance on God.
You are very much mistaken about these people. Please, dear metmom, consider this in your heart.