Prior to 1651, the prevailing view on reproduction was that of the two great ancient Physicians, Hippocrates (460-370 B.C.) and Galen (129-200 AD). (The Gospel writer Luke, who was a physician by trade, would have been trained in the teachings of Hippocrates.) The general theory held that the entirety of a babies identity came from the male, while the woman provided the environment in which the baby grew. If the “seed” for the baby came from the right testicle (which contained the “strong” seed), the child would be a boy, but if it came from the left or “weak” testicle, it would be a girl. Alternatively, there was a theory which said that the woman carried the “weak” seed, while the man carried the “strong” seed and that after copulation, the woman “decided” which seed would win. This, however, was a far less popular theory.
In 1651, William Harvey (yeah, the same William Harvey who discovered that blood circulates throughout the body!) published a book called De generatione animalium, in which he asserted that the preformed embryo resided not in the man, but in the woman. Ex ovo omnes (from the egg comes everything) was the famous slogan from the book. (We should note that in publishing this work, Harvey vindicated Henry VIII, who had executed various wives for not “giving” him a male heir.)
Now a debate raged, between the “old guard” who held that the males held the preformed embryo, and the ovists, who argued for the woman.
Spermatick worms (or animalcules) were first observed by Antoni van Leeuwenhoek ca. 1677. Their exact role in conception was not immediately understood, and indeed, for nearly two centuries, a primary school of thought was that tiny creatures were actually parasites. In fact, the very name we now use still bears this stigma: the organisms came to be called “spermatozoa” (singular: spermatozoon) because they were classified as parasites (entozoa, family prothelmintha) by the Anatomist Richard Owen in 1835.
Anyway, the debate raged between the ovists and the animalculists who asserted that it came from the sperm and was merely nurtured by the female. This debate continued for another 200 years. The mammalian oocyte (the ovum) was not actually observed until 1827 (even though Harvey had postulated its existence in 1651). Mammalian fertilization was not observed until 1875!
Amongst the scientific and medical communities, the ovists generally became the dominant faction. But (and this is very important!) amongst the general population (including the theological faculties of the Universities!), the animalculists remained dominant. Thus, it is completely understandable that almost universally in theological circles, contraception was condemned as sinful, because when a man prevented his semen from reaching its goal, he was killing a pre-formed embryo. Not until after 1875 could this opinion have changed, and frankly, not until this idea had fully germinated (pun intended) would the general public even remotely comprehend the idea.
Thus it should be of no surprise at all that it was not until the Lambeth Conference of 1930 that the first Christian church opted to stop calling (some forms of) contraception a sin.
For further reading, I recommend two books, both readily available at your local University library:
E. B. Gasking, Investigations into Generation, 1651-1828, London: Hutchinson, 1968.
C. Wilson, The Invisible World. Early Modern Philosophy and the Invention of the Microscope, Princeton: Princeton University Press, 1995.