godisgood77 is right on that score and where NFP is preferred to ABC (artificial birth control).
There is still a difference though, between NFP and ABC. The former is by abstinence which does not interfere with the biological function of the body for procreation, the sole prerogative which is God’s.
The latter, as opposed to the NFP, does interfere with our body’s procreation purpose, and the Church opposes to that, as we should not deny our body to function naturally (conception - when the sperm fertilizes the ovum) as how God created it to be.
Bearing in mind too, our body is the temple of God (the Holy Spirit). So it should not be meddled with and made unholy by artificially stopping it from carrying its function in this most sacred process of creating life.
The Church does not prohibit a couple from spacing their family, but if they should, of course even employing NFP they are open to life, then the NFP is the way to go.