First, if the Lord Jesus was the result of the union between Joseph and Mary then He would have been disqualified to occupy the throne in Israel.
Joseph was a descendant of Coniah. Coniah or Jeconiah and his descendants were forever barred from occupying the throne of David (See Jer. 22:24-30 and Matthew 1:12)
Matthew is the genealogy tracing the Lord Jesus from David to Joseph His stepfather. Luke is the genealogy tracing the Lord Jesus from David to Mary His mother. Thus, the Lord Jesus was qualified to sit on David's throne as King because He was also in David's line through her mother. God declared in the Old Testament that the king who would reign in Israel forever must come from David's line though not through Jeconiah.
Second, if the Lord Jesus Christ was the result of the union between Joseph and Mary then He would have inherited a sinful nature from them. The sin nature is passed from parents to their children by natural birth. This would disqualify Him from being the Savior.
It should be noted that Mary was simply a vessel or a container of the human body of the Lord Jesus Christ. The Lord's body was formed miraculously without the union Joseph's sperm cell and Mary's egg's cell. The Lord Jesus was IN Mary but not OF Mary.
Notice:
Mt 1:20 But while he thought about these things, behold, an angel of the Lord appeared to him in a dream, saying, "Joseph, son of David, do not be afraid to take to you Mary your wife, for that which is conceived IN HER is of the Holy Spirit.
