Parallel
Genesis 38:9
Brenton's English Septuagint · Berean Standard Bible
And Aunan, knowing that the seed should not be his—it came to pass when he went in to his brother's wife, that he spilled [it] upon the ground, so that he should not give seed to his brother's wife.
But Onan knew that the offspring would not belong to him; so whenever he would sleep with his brother’s wife, he would spill his seed on the ground so that he would not produce offspring for his brother.