It is stated in the Christian Gospels that Jesus was descended from David and has been Head of his dynastic house since birth, and is thereby the lawful King of Israel. Although Christian faith has it that Jesus was born of the Virgin Mary and, consequently, has no natural human father, he is nevertheless considered to be a Prince of Judah as though Mary's husband Joseph were in fact his father. According to Jewish law a man is presumed to be the natural father of every child his wife bears, unless the contrary is judicially proved. Alternatively he can unilaterally deny paternity, but not when he has already taken paternal responsibility for the child, such as by naming him.
Additionally, in many ancient cultures, including Israelites, genealogy was not considered the same as paternity. For instance, under the Mosaic law, if a man died without offspring and he had a brother, the brother was legally required to marry the widow and the dead brother was considered the father of their first child (Genesis 38). Under such broader views, Jesus could be the legal son of Joseph, and therefore be from the line of David, without being Joseph's biological son and without being adopted.
Also, the New Testament lists two different genealogies for Jesus, one at the beginning of Matthew and one in Luke 3:23-38. Matthew says "A was the father of B, B was the father of C", etc. Luke, however, uses a word that can mean either "biological son" or "descendant", in the form "C was the son of B, who was the son of A". Matthew shows a lineage from David, father of Solomon and Luke shows a lineage through Nathan, a son of David. A common explanation offered by Christian biblical scholars is that Matthew is stating Joseph's line and Luke is stating Mary's line. Under this interpretation, Jesus would be a biological descendant of David through his mother.
http://en.wikipedia.org/wiki/Davidic_line
There are times when my memory serves me right. I think I'll just go home and celebrate this as one of those days.