In honor of which I link to the encyclical,
one of many recent articles on the subject,
and a really good theological review by Elizabeth Anscombe in 1972.
I grew up not knowing why the Catholic Church is against the use of contraception and assuming that I'd use the wonderful Pill myself one day if needed. My first shock on that subject came a few years ago when the man I was dating said, "Did you know that the Pill is abortifacient?" Turns out it works not just by preventing ovulation but by preventing the implantation of eggs that do manage to ovulate and get fertilized anyway. The embryo gets sloughed off in menstruation instead. For those who accept that human life begins at conception, this is an extremely important thing to know, and yet I never learned it, either in health class or in church or from friends or family. (I'm told that it is printed on the informational insert that comes with the Pill, so I hope women read carefully.)
I think that was what led me to read some of the few Protestant writings against contraception (all kinds, not just the kinds that cause very early abortions), and I was soon sold on the idea of NFP as the best way to limit the number of children. This was well before I thought of becoming Catholic, but I'm proud now to be in a church that teaches what's best no matter how she's despised for it. (Frankly, it also makes things easier for me in one very practical way. When I was Protestant I knew if I ever met a great man who wanted to marry me, I'd have to convince him that periodic abstinence is great, really! But now that my definition of "great marriageable man" includes "faithful Catholic", that issue's automatically settled.)
Another thing I didn't know was that the Christian denominations all used to agree that contraception is morally wrong. The anti-contraception laws on the books in America were put there by Protestants. The Anglicans broke ranks in their 1930 Lambeth conference, and in the following decades pretty much every Protestant denomination reversed its teaching. (As for the Orthodox, my understanding is that some of their bishops hold steady against contraception and others waffle.) People assumed the Catholic Church would change its stance as well, and so there was great controversy in 1968 when Pope Paul VI reinforced it instead.
Obviously with the invention of the Pill there was great pressure on the various denominations to say it was fine. I don't think they would all have suddenly agreed that 1900 years of teaching against contraception had been a mistake if not for the social changes in the world at large. This fact by itself proves nothing about whether they were right or wrong, but we can say that absolute truth does not alter with changing circumstances-- if it was inherently wrong in 300 and in 1570, then it's wrong today.
The breakdown in sexual morality precipitated by the Pill also proves nothing about whether its use is inherently immoral. But I would argue that these two circumstances-- that Christians used to oppose contraception, and that abortion and broken families have become terribly commonplace since the opposition ceased-- are powerful reasons for every serious Christian to at least find out for himself and make sure he understands the arguments against contraception.
(Actually, though I say "every serious Christian," Pope Paul VI addressed his encyclical to "all men of good will" and based his arguments not on revelation but on natural law.)
I like having a blog. I can't lecture people with this stuff in normal conversation but this way I can get it off my chest. :)