If I understand the Catholic teaching on contraception correctly (and I'm sure I'll be told if I don't), it isn't derived solely from Scripture, but depends on natural law. The idea behind the "Theology of the Body" -- it seems to me -- is that we should look at the workings of the body, particularly the interaction of male and female bodies in sex, and discern the Creator's intent in His design. From that intent, we deduce what we must do to be in harmony with the Creator's intent for our use of our sexual capacity. Thus we discover laws of God that are not set out in Scripture, are not deduced by good and necessary inference therefrom, but are written in nature.
The danger I see in this approach to discerning God's moral will is that it is not solely dependent on the Word of God, which is God-breathed, or even (if you Catholics will feel more comfortable with this) on the Word of God and the Tradition of the Church, but is instead dependent on some theologian's perception and understanding of nature, at the point in time when he wrote. That understanding of nature is not God-breathed, infallible, or inerrant.
Consider the English words we use for the contents of male ejaculate: semen and sperm. semen is the Latin word for "seed." Sperm is derived from sperma, the Greek word for "seed."
("Contents of male ejaculate" is an awkward phrase, but for the purposes of this discussion, about the derivation and use of the words "sperm" and "semen," it's the only way to be clear.)
In Matthew 13:24 -- "The kingdom of heaven may be compared to a man who sowed good seed in his field." -- "seed" is a translation of the Greek word sperma. Jerome in the Latin Vulgate Bible renders the word as semen. This is the literal use of the word sperma -- the seed of a plant.
In Luke 1:55, the end of the Magnificat, sperma is used to refer to the descendants of Abraham, recipients of God's promises. Obviously, it isn't literal here. Again, the word is rendered in Latin as semen (actually, it's dative case, semini), and in English, it's "seed" or "descendants" depending on the translation.
In Genesis 38:9, the story of Onan, the Vulgate has Onan spilling his semen on the ground, while in the Septuagint, the word sperma is used to refer to Onan's late brother's descendants, but isn't there explicitly as the object of execheen (spill), although it seems intended to be understood, which means that sperma is used here to refer to male ejaculate and metaphorically to refer to someone's heir or descendant.
The fact that these words are treated as equivalent indicates the understanding of human reproduction throughout all but the last 180 years of history. The contents of male ejaculate was thought to be equivalent to a seed, and we know now that that isn't true.
A plant seed needs only nutrients and protection to reach maturity. All the necessary genetic information to make a new plant is already contained in the seed.
That isn't true of the contents of male ejaculate. In order for a new human to develop, a male gamete has to unite with a female gamete. At that point, and only at that point, you have the human equivalent to a plant seed -- the necessary genetic information is complete and only nutrition and protection are required for it to develop into a full-grown human.
But Augustine and Aquinas didn't understand that, nor did Luther and Calvin. They thought the male ejaculate itself contained the seed, complete in itself, needing only the hospitable environment of the womb -- the soil for the human seed. Given that view of reproduction, it's no wonder that ancient and medieval theologians regarded "spilling the seed" as a monstrous act, the destruction -- not the prevention -- of precious souls.
If you have that kind of mistaken understanding of the mechanics of reproduction, it would lead you to draw wrong conclusions about the divine purposes behind those mechanics, which would in turn lead you to wrong conclusions about what God expects us to do in light of those purposes.
So if natural law is the basis for your case against contraception, it isn't unreasonable that in 1930, for the first time, a Christian denomination would conditionally approve the use of contraception, because it had only been 100 years since the human ovum was discovered and only 55 years since the necessity of the union of sperm and egg for reproduction was confirmed.
I wonder what, if anything, orthodox Protestants who lived in this period of discovery, men like Spurgeon, Hodge, and Warfield, had to say on the issue of birth control.
Addendum: After re-reading this, I feel I should clarify a couple of points.
In citing the way the words sperma and semen are used in the Greek New Testament, the Septuagint, and the Vulgate, I'm not meaning to suggest that the Bible is mistaken in its presentation of human reproduction. The words were used by the original author (or by the translator, in the case of the LXX and the Vulgate) in the ordinary way that they were used by their contemporaries. The Bible isn't a textbook on reproductive microbiology.
John Calvin wrote often about God's accommodation to human limitations. "God's accommodation, as Calvin explains, does not so much comprehensively express who or what God is, but rather, puts who or what He is in language and types that we fallen creatures can understand." So just as parents adjust our vocabulary to suit the understanding of our children, so God communicated His word through His prophets and apostles in the language the people of the time and place understood, using words in ways that were meaningful to them.
God makes plain His expectations of His people in Scripture. We should heed and obey what is taught there, but I don't believe we should enslave ourselves to uninspired and inaccurate views of nature and science and the uninspired commandments derived therefrom.