> Genesis 38:9-10 But Onan knew that the offspring would not be his; so whenever he lay with his brother's wife, he spilled his semen on the ground to keep from producing offspring for his brother. What he did was wicked in the Lord's sight; so he put him to death also.

Was God unfair to Onan? Some regard it as ridiculous that God would judge Onan for refusing to impregnate his sister-in-law. However, this overlooks the corporate survival benefits required in the ancient context. Onan committed multiple serious offenses, any one of which was worthy of God's judgment:

1. He refused to carry out his responsibility as the brother of the deceased. In that social situation, for Tamar to have a child was the only way to ensure an inheritance and survive. Ancient protocol demanded that the nearest male kin provide an heir (later institutionalized as levirate marriage).
2. Onan not only refused, but repeatedly refused. The text implies whenever Onan lay with Tamar. He was a repeat offender and had plenty of chances to fulfill his obligation.
3. Onan pretended that he wasn't refusing. If he was not willing, he could have said so and suffered the disgrace of not helping a helpless widow. Instead, he chose to enjoy conjugal relations while avoiding the obligation.
4. Onan was setting it up so that he would get the inheritance, leaving Tamar penniless and helpless. If Tamar had no heir born to her, the family's inheritance would go to Onan.