I expect someone will correct any mistakes in this telling[1], but basicaly the Catholic doctrine against contraception and masturbation are both based on the Biblical story of [link|http://m-w.com/cgi-bin/dictionary?va=onanism|Onan]. He was chastised for "spilling his seed on the ground." This, ever since about the 14th century, has been interpreted to be critical of "wasting" the potential life in the sperm.

During all the study leading up to the Vatican Council in 1976, the Catholic Church had everyone from Cardinals on down to secular scholars studying the Catechism to determine the biblical, theological or other basis for all doctrine. It turned out that the original point of the story of Onan had nothing to do with the "wasting" of the seed.

In the society that Onan lived in, the custom after the death of a married man with no heirs was for his brother to take the widow as a wife to keep the property in the family. Otherwise, whoever married the woman would take all her holdings. Onan, who was already married (betrothed? can't remember which) didn't want to but was pressured. At the last minute he backed out, metaphorically and otherwise, thereby "spilling his seed ..."

So the moral was an exhortaion to put your family's continued wealth ahead of your own desires. Which, barring the particulars of the story, isn't all that bad a moral. But in the 14th century, the pope created the alternate moral as a way of increasing membership in the church.

Fast forward to 1976, and the pope is in an awkward position. He can either go along with an interpretaion that he knows is contrary to the original intent, or he can proclaim that every pope for the past five centuries has perpetuated a lie. Considering that one of the teachings of the Catholic Church is the infallibility of the pope, this second option would call into question any pronouncement the church might make. Under intense pressure from the Cardinals, he left the interpretation in the Catechism unchanged.

This is just one of the many examples I heard in a radio interview with the author of a book about the subject.


[1] Working from memory, so please excuse any minor flaws.