8 Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” 9 But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. 10 What he did was wicked in the Lord’s sight; so the Lord put him to death also.
Genesis 38:8 - 10
ESV - 8 Then Judah said to Onan, "Go in to your brother's wife and perform the duty of a brother-in-law to her, and raise up offspring for your brother. 9 But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the semen on the ground, so as not to give offspring to his brother.
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As I understand the customs of the time, when a married man died without having had children, his unmarried brother was obligated to marry his widow and have children with her. However, the children would legally be the offspring of the deceased brother. (God later codified this requirement in the Mosaic Law (Deuteronomy 25:5). The Sadducees referred to this same custom when questioning Jesus about the Resurrection (which the Sadducees did not believe in)(Matthew 22:23-33).) In the account from Genesis cited in the question, Er (a son of Judah, and grandson of Jacob) was slain by God for his wickedness. Er's brother Onan was then expected to marry Er's widow (Tamar) and have children with her on behalf of his deceased brother. However, since the children would not be legally his, Onan was unwilling to fulfill this obligation. Therefore, when he had sexual relations with Tamar, he deliberately avoided the possibility of making her pregnant by discharging his semen on the ground. This prompted God to slay him (as He had done with Er) for failing to perform his fraternal duty. Since Judah was unwilling to allow another of his sons to marry Tamar (for fear that that next son might be killed also), Tamar eventually resorted to deceit by disguising herself as a harlot, and tricking Judah himself into having sexual relations with her, as a result of which she gave birth to male twins (Perez and Zerah), with Perez becoming an ancestor of Jesus (Genesis 38; Matthew 1:3; Luke 3:33). (Sometimes, the account of Onan is cited by Bible commentators to condemn the practice of masturbation (or "onanism", as they may call it). However, such an interpretation of Onan's actions is not an accurate portrayal of what he was doing.)
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