I am trying to discover why Jesus identifies himself with the king David and why the Bible elevates the kingdom of David to such a high model and pattern for the people and purposes of God. I thought that if David was that model and was a man after God's heart he would have some strong parameters to sustain the Christian faith as it relates to the kingdom of God. When I found out that he had many women I felt that he was shallow and poor spiritually and morally (2 Samuel 5:13). Then I got disappointed and the credibility of the plan of God as related to the city of David diminished. When some people say that David didn't have full revelation of the purposes of God because the Bible is a progressive revelation, I though it was a weak excuse because a simple command given in Genesis 2:24 and Deut 17:17 was enough to understand that a man should have only one wife. No special progressive revelation is necessary to understand a simple and clear command. They may call it polygamy and not adultery, but it doesn't deal with the nature of the relationships but only the legal status. And they assume that since it was another legal way to be married and also have concubines, it has nothing to do with immorality. The whole excuse sounds ridiculous and it still leaves the kingdom of David and all its reputation to be in check. So, the purpose of God in using David and calling him a man after His own heart lost value for me.

Answer

First, always remember that God's plan and purpose is perfect, and that His Word (the Bible) is perfect and without error.  Thus we know that King David was picked, by God, for the role he held before the creation of the universe.  God knew exactly what David would be and do, and picked him anyway, for it served God's purposes.
 
Secondly, David was a man, who struggled with sin and temptations like we do.  How would we like our lives and sins spread across the pages of Scripture?  How about if God allowed our daily thoughts and secret sins be manifested to everyone--how would we fair as a Christian example and testimony?
 
Third, we do have the complete revelation of God--far more than David did, so it's easy for us to look back on his life and be critical.  We also live in a culture that prevents us from have multiple wives and concubines, let alone the fact that we couldn't afford it.  If we lived in that culture and it was legal to have multiple wives and concubines and we could afford it, then I think a number of Christian men would be easily tempted to do so--simply look at what Christians do today, that are legal, but sinful.
 
Lastly, God said about David, "I have found David son of Jesse a man after my own heart; he will do everything I want him to do."  [Acts 13:22]
 
Thus we see God picking David to be King because he knew David's heart was devoted and sold out to Him.  Yes, God knew David would sin with Bathsheba, and take multiple wives and concubines, but God also knew that the intent of David's heart was to love God with all his heart and be obedient, even though, like us he would fail to have perfect obedience.  God loved the intent of David's heart to serve Him fully--that's what God wants from each one of us--a heart that is determined to avoid sin and serve Him completely.