I understand why it seems bizarre. But God giving grace to people in different periods never change the lawfulness or unlawfulness of a subject matter. This is my take away from Romans 5. No Mosaic law was given until Moses which means sin was not imputed to those people. Nonetheless, sin still existed and everyone still died. The consequence of sin always remains. In all those stories of the people who had polygamous or incestuous relationships, all kinds of trouble and problems occurred.
The OT for Christians is an example for us, not a covenant. We are not under the law but grace. So the OT stories warn us of consequences through those examples if we choose to sin. In Christ sin is not imputed, but the consequences remain. For those who think grace is license to sin, as in the sloppy agape crowd, they misunderstand the purpose of grace. Grace is given for repentance. As we realize those sinful choices we make, grace encourages us to repent. Paul tells us that it is the goodness of the Lord that leads us to repentance. Jesus said that if we love him we will obey his commandments. This does not mean our obedience proves our love, but rather, our motivation for obedience is because of our love for Him.
Much confusion is created when we try to understand God, and what He has done through the ages, with our natural understanding. It takes spiritual understanding which means we have to understand the character of God Himself to understand his motivations and purposes.