Genesis 38:9

9 But Onan, seeing that the offspring would not be his, went in to his brother's wife, but let his seed go on to the earth, so that he might not get offspring for his brother.

Genesis 38:9 Meaning and Commentary

Genesis 38:9

And Onan knew that the seed should not be his
Should not be called a son of his, but a son of his brother Er; this is to be understood only of the firstborn; all the rest of the children born afterwards were reckoned the children of the real parent of them; this shows this was a custom in use in those times, and well known, and was not a peculiar case: and it came to pass, when he went in unto his brother's wife;
to cohabit with her, as man and wife, he having married her according to his father's direction: that he spilled [it] on the ground, lest he should give his seed to
his brother:
lest his brother's wife he had married should conceive by him, and bear a son that should be called his brother's, and inherit his estate; and this is the sin, which from him is called Onania, a sin condemned by the light of nature, as well as by the word of God, and very prejudicial to mankind, as well as displeasing to God, as follows:

Genesis 38:9 In-Context

7 Now Er, Judah's first son, did evil in the eyes of the Lord, so that he put him to death.

8 Then Judah said to Onan, Go in to your brother's wife and do what it is right for a husband's brother to do; make her your wife and get offspring for your brother.

9 But Onan, seeing that the offspring would not be his, went in to his brother's wife, but let his seed go on to the earth, so that he might not get offspring for his brother.

10 And what he did was evil in the eyes of the Lord, so that he put him to death, like his brother.

11 Then Judah said to Tamar, his daughter-in-law, Go back to your father's house and keep yourself as a widow till my son Shelah becomes a man: for he had in his mind the thought that death might come to him as it had come to his brothers. So Tamar went back to her father's house.