Our commentary on this passage can now be summarized. The cumulative weight of the evidence - the structure and sexual explicitness of the text itself and the much greater severity of Onan's punishment than that prescribed for levirate marriage infringements in Deuteronomy 25: 5-6 - leads us to conclude that while Genesis 38: 9-10 very probably includes disapproval of Onan's lack of piety toward his deceased brother, it is nonetheless the unnatural sex act in itself which is presented as the most gravely sinful aspect of this man's treatment of Tamar - the aspect for which God cut short his life. If the inspired author, while knowing the same historical facts, had evaluated them in the way most modern exegetes would have us believe he did (i.e., with complete moral indifference toward Onan's contraceptive act as such), then we would expect quite different wording: "spilling the seed," being irrelevant to the author's interest and purpose on that hypothesis, would probably not even have been mentioned. Instead, we would expect to be faced with an account stating more discreetly that even though Onan took Tamar legally as his wife, he refused to allow her to conceive, so that God slew him for his "hardness of heart," his pride, or perhaps his avarice (in wanting his brother's property to pass to himself and his own sons).
Thus, the traditional interpretation of this passage as a divinely revealed condemnation of contraceptive acts - not as a provision of mere posititve law (cultic or disciplinary) given temporarily for a specific ancient cultural context, but as a particular manifestation of that divine will for the entire human species which had been revealed through nature ever since the Creation - must be seen as supported by serious exegetical arguments. Indeed, quite apart from those arguments, and even without any appeal to the Catholic theological principle that Church tradition must be our guide to the interpretation of Scripture, a purely historical awareness of the unanimity of Jewish tradition on this point highlights how implausible and anachronistic is the view we are criticizing. That view involves the gratuitous suggestion that the ancient author of Genesis 38 was a lone 'liberal' who, in contrast to every other known Jewish commentator until recent times, was unaccountably permissive about unnatural sex acts while at the same time, paradoxically, showing himself (and God) to be unaccountably severe in regard to infractions of the levirate mariage custom.
The witness of Christian as well as Jewish tradition on this point should be emphasized in conclusion. That Onan's unnatural act as such is condemned as sinful in Gen. 38: 9-10 was an interpretation held by the Fathers and Doctors of the Catholic Church, by the Protestant Reformers, and by nearly all celibate and married theologians of all Christian denominations until the early years of this century, when some exegetes began to approach the text with preconceptions deriving from the sexual decadence of modern Western culture and its exaggerated concern for 'over-population.' Sad to say, these preconceptions have since become entrenched as a new exegetical 'orthodoxy' which can no longer see even a trace of indignation in this passage of Scripture against intrinsically sterile forms of genital activity as such.
(Fr. Brian Harrision- Sin of Onan Revisited 1996)