@bookofesther: While I see others have already addressed your points, I also want to ask how you could possibly know what the “majority” of NFP users have as reasons. Isn’t such an assertion itself coming close to calumny? Hmmmm?
further, you have, as I understand it, affirmatively misstated the criteria for the employment of fertility awareness, as has already been pointed out. “Dire” is not the status that must be encountered before fertility awareness may be used, but rather serious and just more accurately capture what the Church teaches. Perhaps you could cite some authority for your assertion that a “bishop’s dispensation”, whatever that means in this case, would be necessary? Because otherwise, I think this is just your personal opinion. I think you acknowledge as much by saying it “should” be a sin.
Humane Vitae set out the criteria for responsible parenthood, and it is up to the couple to determine, prayerfully, if they are being called to have a(nother) child in the particular circumstances of each month.