While Jacob was living in that land, Reuben had sexual intercourse with Bilhah, one of his father's concubines; Jacob heard about it and was furious. Jacob had twelve sons.
These are the descendants of Reuben, the oldest of Jacob's sons. (Because he had sex with one of his father's concubines, he lost the rights belonging to the first-born son, and those rights were given to Joseph.
Now, it is actually being said that there is sexual immorality among you so terrible that not even the heathen would be guilty of it. I am told that a man is sleeping with his stepmother!
They want to look for nothing but the chance to commit adultery; their appetite for sin is never satisfied. They lead weak people into a trap. Their hearts are trained to be greedy. They are under God's curse!
This is what he says in all his letters when he writes on the subject. There are some difficult things in his letters which ignorant and unstable people explain falsely, as they do with other passages of the Scriptures. So they bring on their own destruction.