Is practicing natural family planning 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 a mortal sin—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).