If you believe there is no moral issue with avoiding pregnancy in and of itself, then it becomes a question of the means employed in that avoidance…as you can plainly see from the paragraphs I quoted from Casti Connubii, the primary end of the conjugal act is procreation…the good of the spouses, mutual love, etc., are secondary aims…having intercourse during times of suspected infertility to avoid pregnancy SUBORDINATES procreation to those secondary aims. And a lot of effort goes into that avoidance - charts, temperatures, mucus measurement…
The Church does not say how many children you have to have. It does say that you need to care for the ones you have, and for yourselves.
The Church says that you need 1) to be open to children; but 2) does not say that you cannot space them out to what you feel is reasonable, following the will of God as best you can discern it, and 3) that sexual relations have two purposes - children, and the mututal relationship of the spouses. God Himself did not make women fruitful 24/7/365; He gave them a cycle. There is a misunderstanding of NFP if it is considered birth control as ABC is considered birth control; one using ABC is doing two things - the first is an act to frustrate the natural end of the sexual act - conception - which is the use of either the Pill or a mechanical method of preventing pregnancy, and the second is the sexual act.
NFP does not do anything to frustrate the natural end of the sexual act, as it does not render the act infertile. Having sexual relations during the time that the woman is infertile is a natural part of God’s plan - he gave us the cycle, we didn’t invent it or cause it. So if one is using NFP or is not using NFP, the results are the same during that time; the womna will not conceive. Having intercourse during that time has never been consdered immoral in and of itself.
What you seem to be confusing is that a couple can be open to life, but not right now. There is no moral requirement that a couple have intercourse during a woman’s fertile period.
Because NFP requires sacrifice on the part of both parties - the period of fertility is normally the period of time the woman is most sexually interested in the male, so she has to agree to give up intercourse during that time, and the male is generally interested most of the time, so he too has to give up intercourse, it is highly unlikely that two people who view sexual realtions selfishly are going to engage for any length of time in NFP.
Given this sacrifice, it is possible that a couple may adopt an attitude that is not open to children; but given that true sacrifice is based on love, and openness to children is based on love, it is a bit of a stretch to think that much of anyone is using NFP out of a contraceptive mentality. “Not having a child randomly” is not the description of a contraceptive mentality. The description of a contraceptive mentality is a “me first” attitude.
A good source of information about NFP is from One More Soul, and if you truly want to know what the mind of the Church is about this, I would suggest that you google it and get their catalgue.
The short answer is that the Church, from at least Pius XII (and I think it goes back to Pius X, but I am a lousy historian) has approved spacing children by natural means. Each couple needs to determine what they can legitimately deal with; some are graced with more materially; others with patience; others with better mental and/or physical health. All of this and more enters into the determination of when and how many children to have.
But your comment about the means is not in the mind of the Church. It is not an issue of avoiding pregnancy; it is an issue of when and how often one is pregnant. The Church encourages large families; that is entirely different from requiring them.