Genesis 38:8-10

"And Judah said unto Onan, Go unto thy brother's wife, and perform the duty of a husband's brother unto her, and raise up seed to thy brother. And Onan knew that the seed would not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest he should give seed to his brother. And the thing which he did was evil in the sight of Jehovah: and he slew him also."

There are a number of extremely interesting questions that hinge upon what is revealed in this passage.

1. From this it appears that the social custom of brothers raising up children to a deceased brother's name through marriage to his widow is much older than the Mosaic Law, which elevated this custom to the status of a divine command about four hundred years afterwards (Deuteronomy 25:5). This does not indicate a late date for Genesis; for, "The existence of the practice has been traced in different forms in Indians, Persians, and other nations of Asia and Africa."[14] As a matter of fact, "The custom of levirate marriage prevailed widely in primitive times."[15] The family or tribal ownership of land required this arrangement in order to assure a more equitable distribution of real estate. Without such an arrangement, the death of a childless man would have transferred his estate to another branch of the family. In this example of it, if Onan had been willing to give Tamar a child, the child would have inherited an estate which would have reverted to Onan in the absence of any heir to his brother Er. This would have substantially reduced the wealth which Onan would have received, since his brother Er was the firstborn and would have received the double portion.
   
2. It should be particularly noted that the word "seed" is used with two different meanings here. It is used for offspring in Genesis 38:9a, and a physical emission in Genesis 38:9b. A similar diversity is seen in God's promise to give Abraham innumerable "seed," (offspring) and that in his "seed" (singular) all the families of the earth shall be blessed. Although our version uses "it" for "seed" in Genesis 38:9b, the true meaning is "seed."[16] "The same Hebrew noun [~zera`] in this verse is used both in its literal sense and in the secondary sense."[17]
   
3. Onanism is allegedly founded here. "Catholic theologians, lacking any authority for their extreme position on birth control, have taken this ancient story of Onan, distorted its meaning by declaring that Jehovah slew Onan for his "coitus interruptus", and inflated this interpretation (!) into a whole system of social hygiene for the 20th Century."[18] If this event has any moral at all, it is that every man who refuses to marry his brother's widow and have a child by her should be killed. That, of course, is what happened to Onan. His willful disobedience to his father, his shameful disregard of the rights of his deceased older brother, his heartless fraud perpetrated against Tamar who desperately desired a child, and his selfish greed in hoping to have a bigger estate himself by cheating Tamar ... that was his sin. As Hobbs stated it: "He was condemned, not just for spilling his seed, but for doing it in order to avoid his marital responsibility."[19] "This has nothing to do with masturbation (`onanism'). It was selfish greed."[20] Why is it that such sins are not punished by death today? This sin of Onan required the fatal judgment of God because it could have thwarted the proper foundation of the Messianic family, and was therefore a threat to God's purpose of redeeming mankind. There can be no doubt, that if a similar threat existed today, the judgment of God would be executed in such a manner as to remove it.
   
4. "The thing which he did..." This should not be construed as reference to a single act. "The verbs in the second and third clauses of Genesis 38:9 are frequentative and should be translated, `whenever he went,'"[21] thus indicating, not a single act, but a long sustained purpose. Payne read it properly as a declaration that Onan, "persistently and maliciously cheated Tamar of her legal rights."[22]