Go in, listen to , lie beside, be with

Genesis 38:7-10 ESV

But Er, Judah's firstborn, was wicked in the sight of the Lord , and the Lord put him to death. Then Judah said to Onan, "Go in to your brother's wife and perform the duty of a brother-in-law to her, and raise up offspring for your brother." But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the semen on the ground, so as not to give offspring to his brother. And what he did was wicked in the sight of the Lord , and he put him to death also.

Genesis 39:5-10 ESV

From the time that he made him overseer in his house and over all that he had the Lord blessed the Egyptian's house for Joseph's sake; the blessing of the Lord was on all that he had, in house and field. So he left all that he had in Joseph's charge, and because of him he had no concern about anything but the food he ate. Now Joseph was handsome in form and appearance. And after a time his master's wife cast her eyes on Joseph and said, "Lie with me." But he refused and said to his master's wife, "Behold, because of me my master has no concern about anything in the house, and he has put everything that he has in my charge. He is not greater in this house than I am, nor has he kept back anything from me except yourself, because you are his wife. How then can I do this great wickedness and sin against God?" And as she spoke to Joseph day after day, he would not listen to her, to lie beside her or to be with her.

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I won't pretend to have all the answers when it comes to understanding these chapters in Genesis (38-39).
However there are hints and themes that point us towards God's intent.

May I remind you that proliferation of the human race has been job one since man's creation. And although the blessing of offspring has been particularly promised to Abraham's family that promise has required a lot of faith. And proliferation when it does happen seems to be mired in family conflict, deceit, and immorality.

But the key to gaining clarity in this mottled ancestry is to look for God, and God's promise. Who is trusting God even when it appears to be counter productive in realising the promise. See God comes before even the promise. Hearken back to Abraham and Isaac with the lamb. Our experience of blessing should never need to be at the cost of God's glory. We can be faithful to God without forcing our self interest at the cost of regard for God's word.

To be honest I don't have all the answers about these two chapters. But I know that we are meant to notice the contrast. Two of Judah's sons are killed by God. Joseph is blessed. But it's not quite that simple. Joseph is a slave. Joseph is in Egypt. Joseph has no children.
Judah's sons Er and Onan are wicked and they are not blessed.
Judah, Er, Onan, and Joseph all have to make a choice about sex. Only Joseph's abstinence policy is blessed. But again blessing includes prison, slander, and loss. In Joseph's life the way up is very much down.
The Lord blessed Joseph's regard for God's way. Judah, Er, and Onan did not consider God's plan.

Do you?
Are you willing to be blessed while in prison, slandered for doing right, or sold into slavery because you are relating God's plan revealed to you?

Ultimately this is a story about Jesus. Can you see Jesus? How would Jesus explain his Father's plan to rescue the world through his humiliation and  death in these few chapters of Genesis?