Is NFP morally acceptable?

Yes, all the major religions including the Catholic Church accept the use of natural family planning when couples have a sufficiently serious reason for spacing babies or family limitation.

On the other hand, the teachings of the Catholic Church, some Orthodox churches, some part of the Judaism and an increasing number of Protestants are opposed to sterilization and the use of contraceptive or abortifacient drugs and devices.   It should be noted that intrauterine devices and birth control drugs (both the pill and implants) can cause very early abortions.