Other than papal decrees, you can’t prove that it IS spiritual poison? I hope if you are willing to call contraception “spiritual poison,” that you are NOT using NFP to avoid conception on a regular basis. I remind you that the Catechism and the Church is very clear that NFP is NOT to be used for a prolonged period of time as most of these orthodox folks believe. NFP is supposed to be used only “periodically” in SPACING out children for “just reasons.” I would be that you have few just reasons yourself like most of us. What is a “just” reason? Prolonged mental or physical illness, extreme poverty, etc. I can think of several other “just” reasons from my perspective. But the Church does not give us much guidance in this area. So, NFP in itself is NOT to be used as a “natural contraceptive.” That being said, to avoid the “spiritual poisons” of contraception and to make sure we are living in total charity, self-giving, only having a “just reason” to use NFP, that would mean, and let’s be honest, that most of our lives we DON’T really have many “just reasons” so, if one were to marry at 25 years old and remain fertile until, let’s say, 47, and only HALF of your marriage you had “just reasons,” you would have 11 kids if you were very fertile. And that’s only if HALF of your marriage you had these very serious reasons. Well, more likely 75% of your marriage you would not have good enough reasons to use NFP so you would, if fertile, have 17 kids.
So, to be a good Catholic, you better plan on having 10-18 kids or so. Why? Because, as stated, NFP is NOT a natural form of birth control. The Church very plainly tells us that it is to be temporary in cases where, “for just reasons,” we can’t properly afford to or are unable to care for a child. We know that, for most of us, such cases are rare, so, you’ll be having one large litter! But, even most orthodox Catholics who love to brag that they don’t use “spiritual poisons” of birth control do INDEED use NFP as just that, natural birth control, feeling proud and justified that they are holy and artificial birth control users are doomed sinners taking poisons. In reality, NFP is a sin if used in a prolonged manner as natural birth control.
Such matters are not simple and hard for just about everyone. Very few Catholics follow the teachings of the Church on this NFP issue, even NFP users!