I said: “Why doesn’t Armstrong quote from a major Jewish commentary on Genesis like Sarna's?
He said: “Why don't you quote from *any* Jewish source, rather than simply assert things about the Jews and give us a bald reference citation? At least I gave some substantiation of my opinion.”
Be careful what you ask for!
At present, Nahum Sarna has written the standard Jewish commentary on Genesis. As he and Chaim Potok say in the introduction to the series:
“In this century, there has been a revival of Jewish biblical scholarship; Israeli and American scholars, in particular, concentrating in the fields of archaeology, biblical history, Semitic languages, and the religion of Israel, have opened exciting new vistas into the world of the Scriptures. For the first time in history, we have at our disposal information and methological tools that enable us to explore the biblical text in a way that could never have been done before. This new world of knowledge, as seen through the eyes of contemporary Jewish scholars and utilizing at the same time the insights of over twenty centuries of traditional Jewish exegesis, is now available for the first time to a general audience in The JSP Torah Commentary."
Moving on to Sarna’s specific comments:
THE LEVIRATE OBLIGATION (38:6-11)
The death of Er without a son made Onan subject to the levirate law. Marriage between a man and his brother’s wife is strictly forbidden in the Pentateuchal legislation of Leviticus 18:16 and 20:21. The only exception to the prohibition occurs when the brother dies without a son. According to Deuteronomy 25:5, a man has an obligation to his widowed sister-in-law. This institution is known in Hebrew as yibbum, or “levirate marriage.”
The levirate institution long antedated the Pentateuchal legislation. In fact, it is widely documented in one form or another in several extrabiblical sources. The compendium of laws from the Middle Assyrian Empire (15-14C BCE)…The Hittite laws (14th-13C BCE)…A contract from the town of Nuzi from the middle centuries of the second millennium BCE…
[Thus], she would be assured of livelihood and protection.
The surviving brother became a surrogate for the deceased husband who posthumously gained a child, socially acknowledged to be his progeny and heir.
The callous refusal of Onan to perpetuate the line of his brother may have been due to a lack of sense of duty to the dead. An even more powerful motivation would have been the fact that with the death of the first-born, Onan inherits one-half of his father’s estate. However, should he provide an heir to his brother, his portion would be diminished.
Genesis Rabba 55:5-6 understands that he practiced a primitive form of birth control through coitus interruptus…Clearly, society at this time had made no provision for voluntary renunciation of the levirate duty as is found in Deuteronomy 25:7-9.
The text does not make clear specifically why Onan incurs divine wrath. The development of the narrative favors the explanation that it is due to the evasion of his obligation to his dead brother rather than because of the manner in which he acts. By frustrating the purpose of the levirate institution, Onan has placed his sexual relationship with his sister-in-law in the category of incest—a capital offense. The unusual emphasis given to the particular socio-legal background of the story clearly shows that the point at issue is the levirate obligation and not the general topic of birth control.
N. Sarna, The JPS Torah Commentary: Genesis (JPS 1989), 266-67.