|Pope Paul VI, author of 'Humanae Vitae'|
The current argument of the Roman Catholic Church against contraception is easier to understand, so let me begin there. You can actually read their explanation in about 30 minutes or less in the papal encyclical Humanae Vitae, which Pope Paul VI promulgated in 1968. Let me quote the crucial sentences:
The Church...in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life.Marital sex--and there should be no other kind, from the church's point of view--must always be open to conception and new life, because of the 'law of nature'. Therefore, as another Catholic website put it,
This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act....The fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason.
Christians have always condemned contraceptive sex. The early Fathers recognized that the purpose of sexual intercourse in natural law is procreation; contraceptive sex, which deliberately blocks that purpose, is a violation of natural law.It's pretty much as simple as that, as least for Catholic bishops. Because it opposes God's will as expressed and known through natural law, any form of contraception (with the exception of the rhythm method) is 'intrinsically evil', both inside and outside of marriage. The church has no alternative but to oppose by all its means--including legal means and the use of the state power--the use of contraception by everyone at all times.
While most Catholics today are probably familiar with this argument, they probably don't agree with it--studies show that the vast majority of Catholics (anywhere from 75-98%, depending on the study) approve of and use contraception. Non-Catholics however, especially in non-Catholic countries like the US, are probably no longer familiar with this Catholic teaching and need to be reminded of its 'severity'.
For most non-Catholic Christians, the third decade of the 20th Century seems to have been the turning point when it comes to their view of the morality of contraception. Before that time, most Protestant denominations opposed contraception. After that time, most Protestant churches left the issue of contraception up to their members to decide on an individual basis, with their personal conscience being their moral guide. Why this huge shift? Well, it's difficult to know for sure, but here are a few possibilities.
First, since the traditional Christian opposition to contraception had very little direct basis in Scripture, and since Protestantism is particularly reliant upon Scripture for its theological and moral views, the foundation for Protestant opposition to 'contraceptive sex' was actually quite weak. If it wasn't stated forthrightly in the Bible, then it became much more of an optional issue.
Second, traditional cultural opposition to contraception was closely connected to the status of women in traditional society. As long as women were seen as subordinate to men, as long as women's place was understood to be in the home as wives and mothers, and as long as women did not have political equality, then it was easy to ignore the issue of contraception and 'reproductive rights'. But as soon as women began to struggle to free themselves from these traditional roles and understandings, then the contraception issue would naturally arise as a part of the struggle for women's rights. As the last post showed clearly, Margaret Sanger--the founder of the birth control movement--was also a part of the larger feminist movement.
Third, liberal Protestant theology, which came to dominate most of Protestantism in the first half of the 20th Century, was basically an attempt to reconcile traditional Christian thought and action with the forces of modernity, such as science, reason, and progress. What could be more modern than the struggle for women's right, including access to contraception? It was natural, then, that Protestant Liberalism would seek to accommodate itself to feminism and contraceptive freedom.
Finally, prior to the 20th Century, contraception was difficult and crude, so it wasn't so much an issue for most people. Many people didn't really even understand how sexuality worked, so they didn't know how to prevent pregnancy. Condoms were primitive, largely ineffective, and hardly available. But once new forms of contraception became readily available and inexpensive, with the advent of rubber condoms, effective diaphrams, and in particular, oral contraceptives in the early 60s, it was a far different story. Traditional taboos, and hoary theological doctrines put forward by celibate men in red hats, couldn't contain the flood of interest in, and demand for, contraceptive protection against unwanted pregnancy. In particular, the always pragmatic American Protestant tradition quickly caved in the face of this overwhelming flood.