Recently, I read two conflicting opinions regarding this question. Kimberly Hahn, in her book Lifegiving Love said that, at most, the misuse of NFP is a venial sin. I do not have the quote readily available, but she said something to the effect that since couples would be abstaining, rather than misusing God’s gift (as contraception does), the act of abstaining cannot be mortally sinful. It would be a non-act. However, I just read an apologist’s opinion in the “Ask an Apologist” forum here that asserted that NFP, when used in a contraceptive mentality, is mortally sinful, since there is full consent of the will and grave matter.
My feeling is that Hahn is correct in saying that the misuse of NFP is at most venially sinful. One can use NFP with a contraceptive mentality. But it is still not contraception, since every marital act is still open to life. The argument could also be made that NFP could be used to facilitate a mortally sinful act, such as an unmarried couple using NFP to avoid pregnancy. In that case, I would say the misuse of NFP is not what is mortally sinful but rather the premarital sex that is mortally sinful.
I would appreciate it if someone could clarify this. Thank you.