This line is from David in Psalm 51. If you trace David's genealogy, there is evidence that this is a purely personal statement and not a statement universal to mankind. First of all, the marriage bed is holy. Therefore children conceived within a marriage are NOT conceived in sin. If a person cross references 1 Chronicles 2:15-17, 2 Chronicles 11:18-20, and 2 Samuel 17:25 -- and I suggest using the King James or, more particularly, the New King James, as that is, in this case, most faithful to the original, you will find that David's mother was not Nahash, the mother of Jesse's other children, but, rather was Abihail, the daughter of David's oldest brother, Eliab. In other words, David's father evidently had relations with his own granddaughter and the result was David. This explains why he was so much younger than the others. It also explains why he was not really considered part of the family unit (when Samuel went to anoint the new kind, David was out with the sheep and not considered one of the true brothers). David WAS conceived in sin. This does not mean everyone is, and Calvinists should not use this verse as a 'proof text.'