Can NFP be a grave sin?

I am posting this with regards to a question that was asked last week. I had always heard that NFP could be mortally sinful if used with a contraceptive mindset, or without a sufficiently porportionate grave reason. Fr. Serpa responded that he was in agreement with Kimberly Hahn’s assessment that NFP could at most only be a venial sin. This is quite confusing to me, because of the following quote, “Therefore, to embrace the matrimonial state, to use continually the faculty proper to such a state and lawful only therein, and, at the same time, to avoid its primary duty without a grave reason, would be a sin against the very nature of married life.” This is a quote from Pope Pius XII in a letter to midwives. I respect Fr. Serpa and his answers and advice, but my question is how can something that could possibly be a “sin against the very nature of married life” never be a mortal sin? I would appreciate any clarification that you could provide. Thank you and God bless.

Dear tac,

It’s good that you wrote because it forced me to check further on the matter with one of my Dominican confreres who is a moral theologian. His answer: For a husband and wife to selfishly refuse to have children is a mortal sin—quite apart from whether they use NFP or not. NFP is a technique that of itself is morally neutral.

The key word here is “selfishly.” It is possible for a husband and wife to determine not to have children for reasons that are not selfish, e.g., financial or health reasons.

Fr. Vincent Serpa, O.P.

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