John Calvin Commentary Genesis 38:8

"And Judah said unto Onan, Go in unto thy brother`s wife, and perform the duty of a husband`s brother unto her, and raise up seed to thy brother." — Genesis 38:8 (ASV)

Go in unto thy brother’s wife. Although no law had previously been prescribed concerning a surviving brother marrying his deceased brother's wife to raise up children for the one who had died, it is, nevertheless, not surprising that, by the mere instinct of nature, men would have been inclined to this practice.

For since each person is born for the preservation of the whole race, if anyone dies without children, this seems to indicate some defect of nature. Therefore, it was considered an act of humanity to secure a name for the dead, so that it might be evident that they had lived.

Now, the sole reason the children born to the surviving brother were to be credited to the one who had died was so that there might be no barren branch in the family; and in this way they removed the stigma of childlessness. Besides, since the woman is given as a helper to the man, when any woman married into a family, she was, in a certain sense, committed to the name of that family.

According to this reasoning, Tamar was not entirely free, but was held under an obligation to the house of Judah to bear children. Now, although this did not stem from any rule of piety, yet the Lord had impressed it upon human hearts as a duty of humanity, just as He later commanded it to the Jews in their societal laws.

From this, we infer the wickedness of Onan, who envied his deceased brother this honor and would not allow him to obtain the title of father; and this brought dishonor upon the whole family.

We see that many people allow their own sons to be adopted by friends; it was, therefore, an outrageous act of cruelty for Onan to deny his own brother what is sometimes granted even to strangers. Moreover, he not only deprived his brother of the right due to him, but he chose instead to spill his seed on the ground rather than raise up a son in his brother’s name.