8 Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” 9 But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his semen on the ground to keep from providing offspring for his brother. 10 What he did was wicked in the Lord’s sight; so the Lord put him to death also. Genesis 38:8-10

This is an intriguing passage because of the content and cultural norms of the day. Onan knew what the cultural norm was even though it wasn’t yet codified.

We find that in Deuteronomy 25:5
If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband’s brother shall take her and marry her and fulfill the duty of a brother-in-law to her.

He knew that any child he produced through Tamar would be considered his brother’s offspring, even though biologically it would be his. Normally, Tamar would not be able to inherit any of Er’s possessions, so it would come through the son. Onan knew that. He also knew that if there was no heir, he would inherit Er’s possessions and land.

So, on the one hand, he slept with Tamar but deceived her into thinking she would become pregnant. He would then take his brother’s possessions. Onan was getting the best of both worlds, or so he thought. How long did this go on? We don’t know but it was probably more than one time.

The Lord obviously knew what Onan was doing and put him to death for his selfishness and dishonesty. To us it seems like a harsh penalty for lying and deception. After all, eleven brothers deceived Jacob into believing Joseph was dead.

Recall, however, how the Lord loves to look after widows and orphans, and it may explain a little of His actions on her behalf. Onan was depriving her of inheriting her first husband’s possessions. Widows were often mistreated and abused because they weren’t protected by a man.

In that light, it makes a little more sense why the Lord dealt with Onan so harshly.

Speaking of widows and orphans – two classes of people the Lord loves to protect – his blessings still are available to those who choose to help widows and orphans, as well as foreigners and the poor. No matter where you live in the world, those classes of people still exist. There’s a reason why He singled them out for protection. Unfortunately, as Believers, we’ve turned over some of that responsibility to Government agencies, but we all know how inefficient they can be.

Still, there’s work to be done where you are and with what you have.