Genesis 38:10

When Onan spilled his seed, that was not very wrong. These things were wrong.

• He did not do his duty. He did not produce a child for his brother.

• He was not honest. He pretended to do his duty and he lay with Tamar. He could have said, ‘I will not do my duty. I will not lie with Tamar.’ That would not be so bad because he would be honest. But instead he pretended to Judah and he pretended to God.

• He was selfish. If he produced a child for Tamar, that child would have some of the family’s possessions. Otherwise, Onan would have them because he was the next brother. He wanted those possessions for himself.

v11 Then Judah spoke to Tamar, who was his son’s wife. He said, ‘Stay as a widow in your father’s house. Stay there until my son Shelah is older.’ He was afraid that Shelah would die, as his brothers had died. So Tamar went to her father’s house and she lived there.