The Roman Catholic Church considers any form of contraception a very serious sin. Most people defending the Catholic Church's decision point to the document Humanae Vitae as a document of sound arguments against contraception, without having actually read it or knowing the background of the decision. Here is some information on the background of the document.

In 1963, a Commission was called to study birth control policy and whether it should be changed or not as part of the broader Vatican II reforms called by Pope John XXIII. This commission consisted of two priests, one a diplomat and the other a sociologist, and 4 laymen, a demographer, an economist, and two medical doctors. All were married. The Church's policy on Birth Control had until this time been based on the document Casti Connubii, which condemned it as sinful. The Church made the justification that God had already supplied a way for people to have sexual intercourse without having a baby, simply by following the rhythm of ovulation. When Pope John XXIII died, the commission continued on under the direction of Pope Paul VI. The commission met multiple times, broadening its membership to fifteen men. Up to this point, no one had presumed to recommend altering the Church's teaching on contraception.

Things changed at the fourth session, held in the spring of 1965, when the size of the commission jumped up to fifty-eight, with five women, all appointed by the Vatican as devout and intelligent theologians. They heard arguments from many extremely devout Catholics on how practicing rhythm in sexual relations ruined their relationships, making them obsessed with sex and its mechanics until the time when they were allowed to have it, then waiting in fear for the wife's menstruation, which would sometimes not come. It made the couple resentful of each other because they had to hide their passion and push away from each other until the "appointed time". They heard arguments from doctors who noted that the female's peak of sexual arousment was during the time which had to be marked off-limits, and denying that arousment certainly was not natural, as the Church had argued. The commission finally came to the conclusion that the policy had to be changed on this matter and that the truth could no longer be denied. When the nineteen voting members of the commission convened, the vote was 12 to 7 for changing the teaching on contraception.

This set off alarm bells in the Vatican, since they hadn't expected the commission to actually disagree with them, and all the commission members were demoted to the category of advisers. The commission was reformed with sixteen bishops, again appointed by the Vatican, and the evidence was heard again, as well as new evidence for changing the policy. In the climactic vote of the commission, the results were 9 to 3 for changing the Church's position on contraception, with 3 abstentions.

The Pope was infuriated, and published the document Humanae Vitae to reaffirm Casti Connubii and end the debate. It stated that contraception is a sin because it is intefering with God's plan, and that the only practice a Catholic could use was rhythm, which was the "natural" way. In fact, it was even stated that the practice of rhythm was a better way to have sex, despite all of the evidence and testimonials against this. The Catholic Church considers contraception a sin because Pope Paul VI had a temper tantrum when he didn't get the results he wanted from the commission he appointed himself. Consider this the next time you try to justify the Catholic Church's position on contraception.

In response to quijote:
The explanation in Humanae Vitae does not hold up to the investigation of the council formed of many esteemed members of the Catholic church. Several points don't seem to make any sense at all, for instance, "an act of mutual love which impairs the capacity to transmit life which God the Creator, through specific laws, has built into it, frustrates His design which constitutes the norm of marriage, and contradicts the will of the Author of life." The Church approves of the Rhythm method, which is a schedule designed to prevent pregnancy. Hence, it is a birth control method, and those who follow it are "frustrating His design". Either couples must submit themselves wholly to the will of God, or they must be allowed to make choices that allow them to maintain their relationship in a feasible manner. You can't have your cake and eat it too.
That's a side issue, however. I still maintain my main point that the only reason the Church's doctrine on birth control remains is because Pope Paul VI rejected the decision of the council he appointed as the best judges of this moral question, not once, but twice. His mind was entirely closed on the issue. He didn't want an open, intelligent debate on the merits of each side, he wanted blind justification for the view he already held. When he got more than he bargained for from the council, he pulled the Papal Authority card.

An absolutely fabulous way to treat such a sensitive issue of such great importance to Catholics around the world, don't you think?