Onan spilled his seed on the ground because he refused to fulfill the levirate duty of providing offspring for his deceased brother. According to the custom outlined later in Deuteronomy 25:5–6, when a man died without children, his brother was responsible to marry the widow and raise up offspring in the brother’s name to preserve his family line and inheritance.
Genesis 38:8–9 records the event:
“And Judah said to Onan, ‘Go in to your brother’s wife and marry her, and raise up an heir to your brother.’ But Onan knew that the heir would not be his; and it came to pass, when he went in to his brother’s wife, that he emitted on the ground, lest he should give an heir to his brother.”
Onan’s act was not simply about sexual behavior but deliberate rebellion and selfishness. He wanted the pleasure of the relationship without fulfilling his covenant responsibility. By refusing to produce an heir for his brother, Onan jeopardized the family’s inheritance and disobeyed a duty recognized as binding.
God’s response was severe:
“And the thing which he did displeased the Lord; therefore He killed him also.” (Genesis 38:10)
This shows that God’s judgment was not about the physical act alone but the heart of disobedience and disregard for His covenant purposes. Onan’s sin was rooted in greed, pride, and rebellion against God’s plan for family continuity in Israel.
The account teaches that sin is not merely external but flows from selfish motives and a refusal to honor God’s order. It also underscores God’s concern for faithfulness in covenant obligations, pointing to the seriousness of obedience in matters He has ordained.
