Is practicing natural family planning (NFP) with a contraceptive mentality grave matter? If done with full knowledge and consent, would it constitute a mortal sin?

For a husband and wife to refuse to have children for selfish reasons is seriously problematic—quite apart from whether they use NFP. NFP is a technique that of itself is morally neutral.

The key word here is selfish. 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).

For just reasons, postponing the start of a family or planning the spacing of children, which is what both artificial contraceptives and NFP allow, is not intrinsically immoral. Assuming that the postponement and/or spacing is not done for selfish reasons, prudentially planning one's family may be meritorious. What is intrinisically immoral is indulging in relations but frustrating their natural consequences: artificial contraceptives allow a couple to do this; NFP does not.