Contraception is any action that separates the procreative (life) and unitive (love) aspects of sex. Its methods include the use of hormone pills, condoms, IUDs, spermicides and withdrawal during intercourse to block conception from occurring.
Withdrawal was the contraceptive method used by Onan in the Old Testament. Of Onan, Genesis 38:9–10 relates, "He, knowing that the children should not be his, when he went in to his brother's wife, spilled his seed upon the ground, lest children should be born in his brother's name. And therefore, the Lord slew him, because he did a detestable thing." Contraception has, therefore, traditionally been called Onanism in moral theology.
The Catholic Church's perennial condemnation of contraceptives is found in the 1930 encyclical Casti Connubii (CC). In it, Pope Pius XI writes, "Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who in exercising it deliberately frustrate its natural power and purpose sin against nature and commit a deed which is shameful and intrinsically vicious."
The pope also quotes in the encyclical the fourth century Doctor of the Church, St. Augustine:
Holy Writ bears witness that the Divine Majesty regards with greatest detestation this horrible crime and at times has punished it with death. As St. Augustine notes, "Intercourse even with one's legitimate wife is unlawful and wicked where the conception of the offspring is prevented. Onan, the son of Juda, did this, and the Lord killed him for it."
Pius XI issued this condemnation in response to the approval of contraception that year by the Anglican bishops at their Lambeth Conference. They were the first Christian denomination to condone the limited use of contraceptives within marriage. Thirty-eight years later, Bl. Pope Paul VI presented the unchanging Catholic teaching that contraceptive sex is unnatural and, therefore, always objectively sinful. In his 1968 encyclical, Humanae Vitae, Paul VI writes, "Similarly excluded is any action which either before, at the moment of or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means."
Witness the devastating effects of contraception in Church Militant's Premium show, FBI—The Contraception Deception.