“Can anyone help me understand why... NFP (natural family planning) is sinful?”

NFP

Natural Family Planning is sinful because it’s a scheme that deliberately subordinates the primary purpose of the marriage act (the procreation of children) to other things. That’s forbidden.

Pope Pius XI, Casti Connubii (# 59), Dec. 31, 1930: “For in matrimony as well as in the use of the matrimonial right there are also secondary ends, such as mutual aid, the cultivating of mutual love, and the quieting of concupiscence which husband and wife are not forbidden to consider SO LONG AS THEY ARE SUBORDINATED TO THE PRIMARY END and so long as the intrinsic nature of the act is preserved.”

NFP is a calculated attempt to have marital relations while taking positive steps to AVOID the primary purpose. Besides the fact that it contradicts Church teaching, people who use NFP know it’s wrong. It’s a sinful birth control practice. We have a video about this matter: https://endtimes.video/nfp-a-birth-control-deception/