Just wondering how would you judge Abraham who laid with his handmaiden while married to Sarah?
In the OT, after God married Adam and Eve, God told them to
"be fruitful and multiply" so how could they multiply without having incest either with their children or their sons and daughters married their brothers and sisters?
So in order to populate the earth, sibling incest was the only option. BUT, that did not become a "sin" until many, many, many years later as you can tell from these three verses below.
Num 36:8...
And every daughter who possesses an inheritance in any tribe of the people of Israel shall be wife to one of the family of the tribe of her father, so that every one of the people of Israel may possess the inheritance of his fathers.
Gen 10:32...These are the families of the sons of Noah, according to their genealogies, in their nations;
and from these the nations spread abroad on the earth after the flood.
Gen 19:30-32...Now Lot went up out of Zo'ar, and dwelt in the hills with his two daughters, for he was afraid to dwell in Zo'ar; so he dwelt in a cave with his two daughters. And the first-born said to the younger, "Our father is old, and there is not a man on earth to come in to us after the manner of all the earth. Come, let us make our father drink wine, and we will lie with him, that we may preserve offspring through our father." So they made their father drink wine that night; and the first-born went in, and lay with her father; he did not know when she lay down or when she arose.
Lot's wife became "a pillar of salt" and therefore Lot's daughters had to keep that family line going and therefore it was not a "sin" to have sex with the father. (But how that happened? Let's not go there!)
But in regards to Sarah and Abraham and the handmaid, Sarah doubted God's promise that a child could be born of her due to her age so Sarah told Abraham to have sex with the handmaid so that Abraham's lineage would continue. Then the handmaid got pregnant and had a son which she named Ishmael. Because that was not what God had intended, that union produced the line of the Cananites that were never God's chosen people. So, it did not work well for Abraham to not wait upon the Lord for Sarah to become pregnant by Abraham through which the promise to Abraham was made in oder for the line of the Messiah to come through.