tsk, tsk...
GC, a woman would only inherited something from her father (not from her father-in-law), and only if she had no husband, and only in special cases and after all the men in the family had already received their share.
Onan's brother was the firstborn. Since he died, Onan would inherit the double share - but ONLY if his brother had left no heir. If he had had a son with the widow, the child would have been considered an heir of the deceased brother, and the double share would go to the child.
If he didn't give a child to the widow, the inheritance would go to him. After his father died and he had already got the inheritance, he could have a child of his own.
Last but not least, he could have opted for not marrying the widow, but by marrying her he would prevent his other brother from marrying her and giving her an heir.

GC, I've already answered your question. The Bible is clear that "the times of ignorance God overlooked." That's the only reason why they were saved. The problem is, some of them not only had more than one wife, but they had concubines, too. Do you consider concubinage OK, too?

GC: If the Bible taught what you are asking it to teach (it doesn't), the following Godly men would all have been "adulterers."
Abraham
Jacob
Elkanah
David
Solomon

k: Thinking of at least David, what about murderers, a man after God's own heart? Does that mean murderers go to heaven or David won't be in heaven?

GC: Thankfully, there is forgiveness for even the worst of sins. David repented. He did not continue being a murderer.

But if polygamy is adultery, those polygamists in the lists were adulterers to their dying days, without ever having repented of its "evil." Therein lies a fundamental difference.

I'm not sure how you can see fit to reason it this way. It is obvious that if Onan could not be polygamous, and he would choose to marry the widow to keep her from being married and having an heir by the younger brother, that Onan would have been extremely myopic. It would mean he could never have a child of his own!

That is why he would not have any inheritance to pass along to his progeny--because with no progeny, said inheritance automatically reverts back up the line.

Besides, what a way to live...having no son to care for him in his old age? How short-sighted that would be!

I don't think I understand the "concubinage" enough to say whether or not it was "ok." What I do understand is that "sexual relations" and "marital relations" are alike in God's sight. If one has sexual intercourse with someone else, it is an act of marriage. To God, the two are thereafter husband and wife, even if they have not had a special piece of paper notarized by a judge or notary public.

Do you think God would give laws governing our sins because we were ignorant? Like telling us how to sin fairly?

GC, you must have missed what I said about this:

<<After his father died and he had already got the inheritance, he could have a child of his own.>>

All he had to do was to wait until he got his hands on the money. After that he could have as many children as he wished.

Would it be OK for the men who made faith's hall of fame to go to a prostitute?