Genesis 38:7 - Wesley's Explanatory Notes

And Er was wicked in the sight of the Lord — That is, in defiance of God and his law. And what came of it? Why God cut him off presently, The Lord slew him. The next brother Onan was, according to the ancient usage, married to the widow, to preserve the name of his deceased brother that died childless. This custom of marrying the brother's widow was afterward made one of the laws of Moses, Deuteronomy 25:5. Onan, though he consented to marry the widow, yet to the great abuse of his own body, of the wife he had married, and the memory of his brother that was gone, he refused to raise up seed unto his brother. Those sins that dishonour the body are very displeasing to God, and the evidence of vile actions. Observe, the thing which he did displeased the Lord — And it is to be feared, thousands, especially of single persons, by this very thing, still displeased the Lord, and destroy their own souls.