The penalty for Esau was death? The penalty for refusing to obey the levirate law was death?
No, no Christian has believed this down through history.
Onan's sin was exactly what the Protestant Reformers said it was:
Martin Luther (1483 to 1546) -
"Onan must have been a malicious and incorrigible scoundrel. This is a most disgraceful sin. It is far more atrocious than incest or adultery. We call it unchastity, yes, a Sodomitic sin. For Onan goes into her; that is, he lies with her and copulates, and when it comes to the point of insemination, spills the semen, lest the woman conceive. Surely at such a time the order of nature established by God in procreation should be followed."
John Calvin (1509 to 1564) -
Deliberately avoiding the intercourse, so that the seed drops on the ground, is double horrible. For this means that one quenches the hope of his family, and kills the son, which could be expected, before he is born. This wickedness is now as severely as is possible condemned by the Spirit, through Moses, that Onan, as it were, through a violent and untimely birth, tore away the seed of his brother out the womb, and as cruel as shamefully has thrown on the earth. Moreover he thus has, as much as was in his power, tried to destroy a part of the human race.
John Wesley (1703 to 1791) -
"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, refused to raise up seed unto the brother. Those sins that dishonour the body are very displeasing to God, and the evidence of vile affections. Observe, the thing which he did displeased the Lord - And it is to be feared, thousands, especially single persons, by this very thing, still displease the Lord, and destroy their own souls.
From BIRTH CONTROL AND GENESIS 38:
We can look at Genesis 38 itself to see that the argument that Onan was killed because of his refusing to fulfill the obligation to raise up children is insufficient. This theory that God is punishing Onan merely because he failed to fulfill the Levirate rule makes God capricious. For example, in this very chapter of Genesis, not only does Judah not get punished for doing the very same thing as Onan did, (withholding his son Selah from her), but Selah himself withholds himself from her. Given that Judah himself compounds the problem by making her a harlot, Onan's specific act of destroying seed takes a larger picture. Judah had promised to give Tamar his son to her (v.11), when he was older. Judah himself is deceitful, and he himself, when caught, admits that he is a worse sinner than herself (v. 26). Shelah himself, who was now grown up, (v. 14), also was deceitful, should have taken her as her husband, and raised up children. He did not. Tamar notices this, but no deaths of either Judah or Shelah. Thus, they were all in a sense rebellious, and did not do what they should have. So, what is the difference between Judah, Onan, and Shelah? The only substantive fact is that Onan went into her lawfully as he married her (unlike Judah who went into her unlawfully), but only Onan destroyed the seed. Ultimately any attempt to exclude this as the principle grounds of Onan's death, is a pure attempt at expediency.
Thank you for that Scriptural truth. I learn something splendid every day on this forum.