The bit about the Virgin birth is really pretty interesting to sort through..... If you STRICTLY look at the Davidic covenant - you will notice that the Covenant was tenderered to Soloman and his seed... Soloman accepted, but FAILED to keep the covenant terms because of his idolatry - thus negating the covenant with him and his progeny.... so strictly speaking - Soloman's seed could not be the promised line of the Messiah to inherit the perpetual throne of David..... Soloman's line did have the Legal (Earthly) right to the Crown through the right of Inheritance.... but there was the issue of the Blood curse against Jeconiah.... thus impeding this lineage to actually accept the Throne of David.... So Jesus being born of a Virgin - with Mary being related through David's other son with Bathsheba - Nathan - gets around the problem with Soloman breaking God's covenant with him and his seed..... Jesus is still the blood of David - through a lineage that had NOT broken a tendered covenant with God for perpetual inheritance of the throne (because Nathan had never been tendered a covenant by God for perpetual rulership..). The "Daughters of Zelophad" precedent gave Mary (who had no brothers) and her children right to inheritance in HER FATHER'S HOUSE (the family of David through Nathan) - because she married within her tribe (Judah)... so Jesus had a RIGHT to be offered the Covenant of David.... But... Joseph "Adopting" Jesus as Firstborn would then give Jesus the earthly right to inherit the crown through the legal lineage without having to stage a coup to take it by force.... If Jesus was not Joseph's "Firstborn" - he would have no legal duty to care for Mary.... and notice - at the cross, Jesus officially bequeaths Mary's care to the Apostle John... He could not have legally done that if he had not been "Firstborn".... Confused?