Why did God allow polygamy in the Old Testament?

I’ve been reading a bit into 1 Samuel, and came across something that confused me: > **1 Samuel 25:43 NIV** > David had also married Ahinoam of Jezreel, and they both were his wives. But, in Genesis, we read how one single man unites as one with his single wife, as shown below: > **Genesis 2:24 NIV** > That is why a man leaves his father and mother and is united to his wife, and they become one flesh. So, why would God allow people after his own heart, such as King David, to participate in this act?