John Gill Commentary Genesis 38:9

"And Onan knew that the seed would not be his; and it came to pass, when he went in unto his brother`s wife, that he spilled it on the ground, lest he should give seed to his brother." — Genesis 38:9 (ASV)

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: