The famous Scripture scholar, Fr. Raymond Brown, S.S., stated an important principle for interpreting Scripture: How did the Fathers of the Church understand any given text? He argues that their interpretation should give us basic parameters. They were much closer to the Bible both in time and mentality therefore what they say deserves special consideration, particularly when a text is obscure.
If we were to apply that principle to understanding the sin of Onan the result would be pretty straight forward. The act which God abhors is enjoying sex without accepting its natural consequence: the conception of a child.
In the last three or four decades there has been a tendency to view this text differently. Onan's real sin, many now maintain, was not the spilling of his seed on the ground, but rather his refusal to fulfill the Levirate law which required him to give his sister-in-law Tamar a child after his brother Er* had died. That levirate interpretation seems to have convinced most modern scholars. However, we must ask if there is not something also wrong in the way Onan failed to fulfill the Levirate law. After all Onan's younger brother Shelah did not enable Tamar to have a child, but he was not slain. The crucial difference between Onan and Shelah was that the latter did not "lay" with his sister-in-law and then interrupt the act. (cf. Gen 38:14)
Tamar did finally conceive a son by tricking her father-in-law. Judah's incest was shameful, but it was not punished as severely as Onan's coitus interruptus. All this is jarring to our modern sensibilities. Still Matthew saw some merit in it. Tamar is one of four women mentioned in his list of the ancestors of Jesus!(Matt. 1:3)
The question of Onan's sin has not been laid to rest. The modern interpretation might be a case of what Bernard Lonergan calls "group bias," in this instance the deep rooted conviction that our own culture is not only the latest, but the most advanced and therefore the measure for all others. The Bible's reverence for the "seed" seems outmoded to us, but could it be that we are missing something crucial? Maybe our blind spots need to be challenged by reading the Fathers. C.S. Lewis never tired of encouraging Christians to read ancient authors as an antidote to chronological narrowness.
The Church Fathers were certainly aware of the Levirate law, nevertheless they saw the issue as Onan's willingness to enjoy sex, but not accept the consequences. Still, I hasten to add that the Patristic interpretation of Gen. 38:10 is not essential to upholding the Catholic teaching regarding birth control. Humanae Vitae and the Catechism present that teaching without mentioning Onan.
*"Er, Judah's first born was wicked in the sight of the Lord; and the Lord slew him.," (Gen 38:7) The text does not say in what Er's wickedness consisted.
The above article received a citation in Daily Kos
See also: Sin of Onan Revisted by Rev. Brian Harrison, S.T.D.