To deny the reciprocal total gift of self its true expression in the marital act is to deny a true expression of love, which the Church is just not ok with. You are not expressing love for someone when you are holding back a part of yourself from him or her.
So, then why is NFP ok? Well first of all it’s not always ok. If your motivations are simply selfish and there really is no serious reason why you cannot or should not have another child at this time, the Church does not intend for NFP to become simply a “natural” alternative to artificial birth control. It’s meant to be a way for families who would be harmed financially, or in some other way, by having another child at that time. Also, it should be said that choosing to practice NFP has to be agreed upon by both husband and wife. If at any time either the husband or wife wishes to stop practicing NFP, the other cannot use NFP as a justification for denying his or her spouse their marital right.
Natural Family Planning is not simply a “natural” alternative to artificial birth control because it does not reduce sex simply to a means to achieve pleasure. It’s not that pleasure is bad; it’s just that sex is supposed to be about so much more than pleasure—it’s supposed to be about love (i.e. total gift of self).
So yes, while practicing NFP, you are choosing to abstain from sexual activity during those days on which you are most fertile (which by the way, is usually about 4 days out of the month). But you are choosing to do this with your spouse, whom you love, and whom you would never wish to reduce merely to an object of achieving pleasure for yourself. It’s a more consistent expression of the life-giving love that isn’t afraid of a little sacrifice for the greater good of the family.
