Matthew Poole and continuators on Genesis 38:9

9 And Onan knew that the seed should not be his; and it came to pass when he went in unto his brothers wife, that he spilled it on the ground, lest that he should give seed unto his brother .

Two things are here noted 1. The sin it self, which is here particularly described by the Holy Ghost, that men might be instructed concerning the nature and the great evil of this sin of self-pollution, which is such, that it brought upon the actor of it the extraordinary vengeance of God, and which is condemned not onely by Scripture, but even by the light of Nature and the judgment of heathens who have expresly censured it as a great sin and as a kind of murder. Of which see my Latin Synopsis. Whereby we may sufficiently understand how wicked and abominable a practice this is amongst Christians and in the light of the Gospel which lays greater and stricter obligations upon us to purity, and severely forbids all pollution both of Flesh and Spirit? 2. The cause of this wickedness; which seems to have been either hatred of his brother; or envy at his brothers name and honour, springing from the pride of his own heart.