As for NFP, basically the Church says it's okay because it does not involve undertaking some artificial action to prevent pregnancy; since pregnancy is naturally impossible for some period of time. St. Thomas Aquinas taught contraception was a sin against nature. Since the method realisticidealist discussed is natural it is permitted. However, this does not mean any sex act is licit if it doesn't require artificial contraceptives; it still needs to be the same act (vaginal intercourse between a married man and woman).
And anyone with a brain can see that that's complete BS, and thankfully virtually no married Catholic couple believes that.