You’re referring to Romans 1:3? That’s not what it says. Take a look:
*[God’s] Son, descended from David according to the flesh, but established as Son of God in power according to the spirit of holiness through resurrection from the dead, Jesus Christ our Lord. * (Romans 1:3-4)
What Paul is doing here is identifying who Jesus is. Looking from a human perspective – that is, “according to the flesh” – he’s a son of David. On the other hand, looking from a divine perspective – that is, “according to the spirit of holiness” – he’s the Son of God.
Paul isn’t talking about Mary here; he’s talking about Joseph – through whom, legally, through adoption, Jesus was a ‘son of David’.
Perhaps you might object, “but, Paul uses the word σπέρματος here! I mean, that’s literally “spermatos”! He’s talking about literal sperm!”. No, he’s not. Although the etymology of the English word sperm includes this particular Greek word, that isn’t the definition of the word in Greek. The Greek word σπέρματος means “seed”. But, isn’t that enough? Doesn’t that imply literal, physical sonship?
Nope. Look at Luke 1:55 – he uses the same Greek word to mean descendants: “according to his promise to our fathers, to Abraham and to his descendants forever”.
So, Paul isn’t talking about literal physical relationship here.
I’m not saying Jesus had a natural birth. I’m saying He is David’s Son by adoption and blood.
Summa Theologica — Saint Thomas Aquinas
Objection 1: It would seem that Christ did not take flesh of the seed of David. For Matthew, in tracing the genealogy of Christ, brings it down to Joseph. But Joseph was not Christ’s father, as shown above ([4138]Q[28], A[1], ad 1,2). Therefore it seems that Christ was not descended from David.
Objection 2: Further, Aaron was of the tribe of Levi, as related Ex.6. Now Mary the Mother of Christ is called the cousin of Elizabeth, who was a daughter of Aaron, as is clear from Lk.1:5,36. Therefore, since David was of the tribe of Juda, as is shown Mat.1, it seems that Christ was not descended from David.
Objection 3: Further, it is written of Jechonias (Jer.22:30): “Write this man barren . . . for there shall not be a man of his seed that shall sit upon the throne of David.” Whereas of Christ it is written (Is.9:7): “He shall sit upon the throne of David.” Therefore Christ was not of the seed of Jechonias: nor, consequently, of the family of David, since Matthew traces the genealogy from David through Jechonias.
On the contrary, It is written (Rom.1:3): “Who was made to him of the seed of David according to the flesh.”
Reply to Objection 1: Faustus the Manichean argued thus, in the desire to prove that Christ is not the Son of David, because He was not conceived of Joseph, in whom Matthew’s genealogy terminates. Augustine answered this argument thus (Contra Faust. xxii): “Since the same evangelist affirms that Joseph was Mary’s husband and that Christ’s mother was a virgin, and that Christ was of the seed of Abraham, what must we believe, but that Mary was not a stranger to the family of David: and that it is not without reason that she was called the wife of Joseph, by reason of the close alliance of their hearts, although not mingled in the flesh; and that the genealogy is traced down to Joseph rather than to her by reason of the dignity of the husband? So therefore we believe that Mary was also of the family of David: because we believe the Scriptures, which assert both that Christ was of the seed of David according to the flesh, and that Mary was His Mother, not by sexual intercourse but retaining her virginity.” For as Jerome says on Mat.1:18: “Joseph and Mary were of the same tribe: wherefore he was bound by law to marry her as she was his kinswoman. Hence it was that they were enrolled together at Bethlehem, as being descended from the same stock.”
Reply to Objection 2: Gregory of Nazianzum answers this objection by saying that it happened by God’s will, that the royal family was united to the priestly race, so that Christ, who is both king and priest, should be born of both according to the flesh. Wherefore Aaron, who was the first priest according to the Law, married a wife of the tribe of Juda, Elizabeth, daughter of Aminadab. It is therefore possible that Elizabeth’s father married a wife of the family of David, through whom the Blessed Virgin Mary, who was of the family of David, would be a cousin of Elizabeth. or conversely, and with greater likelihood, that the Blessed Mary’s father, who was of the family of David, married a wife of the family of Aaron.
Again, it may be said with Augustine (Contra Faust. xxii) that if Joachim, Mary’s father, was of the family of Aaron (as the heretic Faustus pretended to prove from certain apocryphal writings), then we must believe that Joachim’s mother, or else his wife, was of the family of David, so long as we say that Mary was in some way descended from David.
Reply to Objection 3: As Ambrose says on Lk.3:25, this prophetical passage does not deny that a posterity will be born of the seed of Jechonias. And so Christ is of his seed. Neither is the fact that Christ reigned contrary to prophecy, for He did not reign with worldly honor; since He declared: “My kingdom is not of this world.”
