I have no question that artificial birth control is an intrinsic evil and thus sinful. Why is Natural Family Planning not considered a sin?
From my understanding, the primary end, or purpose of marriage is the procreation of education of children. I would think NFP would be sinful because is frustrates the primary end of marriage and subordinates it to the secondary purposes of marriage, such as mutual aid and mutual love.
Why is NFP considered lawful by the Church?
God gave us the Church. God also designed our bodies to be fertile at some times & infertile at other times. God gave us intellect as well. The use of NFP puts our intellect and God’s design to work for the betterment of our health and our souls.
NFP does not frustrate the primary purpose of marriage (procreation or the openness to life) b/c it does
nothing to alter the natural state of the marital act. Technically, any form or reason of abstaining from relations could be interpreted as NFP and/or frustrating the marital act. We do not have to engage in the marital act daily, weekly, monthly, etc. Instead we are given the free will, intellect, and fertility windows to discern when to engage in the marital act. We also, through God’s design, do not conceive after every marital act. God does not require us to have children constantly. Instead He asks us to be open to life through natural means at all times. Technically, right now I (and I presume you) are open to life through natural means, even though I’m not engaging in relations. In other words, I’m allowing my body to work the way God designed it with no interference from me - however, there is no chance right now of becoming pregnant.
I guess another way to look at it is this: We aren’t required to run into a burning building to save a stranger. If we do we are reckoned a hero by our neighbors. If we do nothing but call 911, we can still be called a hero. If we do absolutely nothing, but also did not cause the fire, we are not culpable. If we do absolutely nothing, but caused the fire, we are guilty of committing harm.
To put that in NFP terms: We aren’t required to have relations during our fertile (or infertile) times. If we do have relations during our fertile times & conceive, we allow God’s will. If we have relations during our fertile times & do not conceive, we allow God’s will. If we tell our spouse we’re fertile, but we prayerfully decide to abstain, we allow God’s will. If we remove our fertile times (contraception) we do not allow God’s will.
Abstaining from relations during a fertile time can be redemptive if it is a sacrifice you make based on what you feel God is calling you to do at that time. It can be morally wrong if the abstinence is not entered into with regards to God’s will for you & your spouse though. However, in my opinion, that is the minority of couples practicing NFP, not the majority.