YOU are the one claiming that David is teaching Original Sin in verse 5. You believe all men are born with a sin nature and MUST sin. That is an EXCUSE sir. It would be completely out of context in David's confession to speak of Original Sin.
And all you have to do is substitute words and it is plain verse 5 is speaking about his mother, not himself.
5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.
5 Behold, I was beaten in anger; and in wrath did my mother chastise me.
Again, sustitute words for shapen, iniquity, sin, and conceive and it is absolutely obvious this verse is describing the actions of David's mother, not David.
Now, I don't know exactly what was wrong in David's family, but because of his mother he was treated as the black sheep of the family. When Samuel came and asked Jesse to present his sons at the feast, TWICE Jesse refused to present David. When Samuel persisted, it was found that David was out keeping the sheep like a servant, while all his brothers were invited to the feast. For some reason, David was treated very poorly by his father and brothers.
And it is probably not a stretch to believe David grew up feeling inferior, and maybe even a little guilty because of his mother. And this is what I believe David is expressing in Psalm 51. He actually had fulfilled what his family had always told him, that he was a BUM. He was not worthy of the family name, and certainly not worthy to come to the feast with the great prophet Samuel.
But David is not teaching that all men inherit Adam's sin or sin nature. That IS a stretch.
Do you know of ANY aprt from Jesus in the bible who never sinned though?
Do you remember ALL save him have sinned and fallen short of glory of God?
We are born physically into Adam and His sin, we are born estarnged from God, very nature at war against Him...
When do we go from all right with Him to sinners then?