I understand that the original commandment to be fruitful and multiply involves reproduction. However, planning to wait a little bit here and there seems reasonable enough.
So, what is the reasoning or the basis from which the Catholic Church teaches that any artificial birth control is immoral/sinful?
Since sexual intercourse is the defining act of marriage, it's helpful to look at what the Catholic Church says about marriage. Traditionally, Catholic theologians have identified three purposes of marriage, which I present here in no particular order:
Generally speaking, I think the prohibition of artificial birth control follows from the principle that married couples should use their sexuality in a way that always respects all three of the basic purposes of marriage. But the use of artificial birth control implies the ordering of a particular act of sexual intercourse in such a way that it is intrinsically incapable of procreation. Before he was Pope John Paul II, Karol Wojtyla argued further, in Love and Responsibility: that choosing to withhold one's fertility in sex is also to work against the unity of the spouses.
All told then, the Catholic philosophical view could be summed up thus: artificial birth control makes marriage one-dimensional, by deliberately limiting its defining act to the service of the least noble of the three purposes of marriage.
Plenty of objections arise on the way from the principle to the conclusion, of course -- particularly in the articulation of how abstinence-based methods can be morally legitimate while artificial methods and sterilization cannot -- but I'm not sure if this is the right venue for considering and responding to all of those.
One thing I'd like to try to clear up, though, if I might. In your question you said:
However, planning to wait a little bit here and there seems reasonable enough.
Of course, and this is precisely what the Catholic Church teaches, at least as I understand it. The clearest official statement of this point that I've seen is Gaudium et Spes paragraph 50.
The Church certainly does not require its faithful to have as many children as they're capable of, nor does it require every act of sexual intercourse to be aimed at procreation -- just to set aside two very common misunderstandings. One has to bear in mind that one of the purposes of marriage, in Catholic thought, is the "procreation and education [or bringing-up] of children". Every married couple must discern for themselves the right balance between fruitfulness and their responsibility for the children they already have.
The main thing is that any "waiting a little bit here and there" be achieved by morally acceptable means. This is of course what the comments on your question were getting at.
Pope Paul VI issued the Humanae Vitae encyclical in 1968 which provided the basis for not using artificial means of birth control.
Part of the argument is that God designed men and women to be fruitful, and that it is morally wrong to play God and circumvent God's design.
The Roman Catholic Church blames Protestants, specifically the Church of England-sponsored Lambeth Conference of 1930, with opening the door to artificial contraception. Yet is was the Roman Church that in 1853 first "reluctantly" or "with reservations," first among mainstream Christendom approved of any contraceptive practice, the rhythm method (periodic abstinence). The Church's reservation" was as an alternative for wives to offer to their husbands who persisted in the practice of "withdrawal," or "coitus interruptus," called "Onanism." Rhythm was not offered as moral but as less immoral than Onanism. Rhythm allowed semen to be deposited in the vagina in the natural manner while Onanism involves "destroying" (the Hebrew) the semen "on the ground," that is, other than in the vagina. Both Onanism and rhythm, and rhythm's improvements, the Temperature/Thermal and the Symptomatic ("Mucus") methods, are not artificial in any normal sense. Both are "open to life" as the both sometimes fail in their intended purpose, avoiding procreation, as artificial methods also often fail.
Some support for the view can be found in an extrapolation of Psalm 127.
Psalm 127:3 Lo, children are an heritage of the LORD: and the fruit of the womb is his reward.
If children are from the Lord, then birth control is like telling him, "No thanks, we have our own plans". Should this attitude be present with those who seek to follow the Lord?
In my own mind I thought I was addressing the question, for from my investigation of this question I get the picture that few who have addressed it have gone back to the original statement by an official Roman Catholic Church organ. My understanding is that the 1853 and 1880 statements and declarations after CC disagree to a considerable extent with Christian doctrine concerning sexual activity dating to the apostolic era, and with Jewish doctrine and tradition before that.
What happened in 1853, reaffirmed in 1880, was that the Sacred Congregation answered a question concerning birth control. By far the most commonly used method was coitus interruptus (withdrawal), always forcefully condemned by the Christian Church of all sects. [Some Gnostic sects, in particular the Manichees, appear to have understood the rhythm method.] The text of both the 1853 and 1880 statements, and commentaries, are readily available on the Internet. "A history of Catholic theology on contraception" is good. It pops up first on google. The author refers to the 1853 and 1880 answers, then points out that Casti Connubii in no wise approves of any birth control measures. The phrase cited by birth control promoters refers to sexual relations after menopause, and with those who are sterile.
