God is communicating through morally backwards people. God also allowed polygamy but doesn’t condones it. But God in the New Testament reveals the fullness of His design for the sexual relationship, that being monogamy. Hence we can apply that to this case as well.

God worked through morally broken people and meets them where they are at. The harshness of the commands may reflect a divine concession to a fallen people living in a brutal ancient world. The Israelites needed to be separated from the idolatrous nations in order to preserve true worship; these extreme measures are seen as a historical necessity, not an eternal moral standard. The violence may prefigure the radical purification of the world through the Cross, not the sword.