Here is a snippet from a comment series on a previous post that I thought was worth highlighting:
Incest was necessary given the nature of God’s creation of human lineage. And polygamy and concubines run rampant in the Old Testament among those deemed righteous.
Incest is not unnatural in the biological sense. One could, and rightly so, argue that it is a very bad idea today given the degree of genetic mutations. However such genetic factors are not a guarantee nor is our present revulsion at the notion a negation of the biological reality of procreation.
You are correct that polygamy and concubines run rampant in the OT. And many who participated in the practice were considered righteous. However none of them were considered righteous for their polygamy or marital indiscretions. In fact, it is abundantly clear that these men were deeply flawed individuals and only considered righteous through grace on God’s part. So to assume their righteousness incorporated all of their deeds is to commit the basic fallacy of assuming salvation or favor with God is merited through works and not through grace.