I said I wouldnt make this post, yet here I am. I challenged brother d to a debate on his unbiblical ideas, yet he refuses to do a debate on the debate section, so be it we all have our preferences, so here I will show how Jesus is of the flesh of david, but 1st let's address why this is important. The old testament is not explicit in the messiah being son of david (explicit meaning directly stating it) but it is still a truth in the old testament that the messiah will be of david's line.
Isaiah 11:1
And there shall come forth a rod out of the stem of Jesse, and a Branch shall grow out of his roots:
The rod is referring to a messianic figure in the context, and jesse is the father of david, therefore of the line of david.
Jeremiah 23:5
Behold, the days come, saith the Lord, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth.
This verse is more explicit saying that the messiah will be of david as well.
So now let's get into why jesus is descendent of david, since this is a internal critique I will be using bibical answers.
Let's look at Jesus's birth "parent's" Mary, and the holy spirit. We know Mary is married to Joseph (Matthew 1:24) which would make marry of david as well under jewish law (Genesis 2:24) since they became "one flesh" from marriage.
Let's establish joseph is of david as well (matthew 1:1-16)
2 Abraham was the father of Isaac, and Isaac the father of Jacob, and Jacob the father of Judah and his brothers, 3 and Judah the father of Perez and Zerah by Tamar, and Perez the father of Hezron, and Hezron the father of Aram, 4 and Aram the father of Aminadab, and Aminadab the father of Nahshon, and Nahshon the father of Salmon, 5 and Salmon the father of Boaz by Rahab, and Boaz the father of Obed by Ruth, and Obed the father of Jesse, 6 and Jesse the father of King David.And David was the father of Solomon by the wife of Uriah, 7 and Solomon the father of Rehoboam, and Rehoboam the father of Abijah, and Abijah the father of Asaph,[d] 8 and Asaph[e] the father of Jehoshaphat, and Jehoshaphat the father of Joram, and Joram the father of Uzziah, 9 and Uzziah the father of Jotham, and Jotham the father of Ahaz, and Ahaz the father of Hezekiah, 10 and Hezekiah the father of Manasseh, and Manasseh the father of Amos,[f] and Amos[g] the father of Josiah, 11 and Josiah the father of Jechoniah and his brothers, at the time of the deportation to Babylon.12 And after the deportation to Babylon: Jechoniah was the father of Salathiel, and Salathiel the father of Zerubbabel, 13 and Zerubbabel the father of Abiud, and Abiud the father of Eliakim, and Eliakim the father of Azor, 14 and Azor the father of Zadok, and Zadok the father of Achim, and Achim the father of Eliud, 15 and Eliud the father of Eleazar, and Eleazar the father of Matthan, and Matthan the father of Jacob, 16 and Jacob the father of Joseph the husband of Mary, who bore Jesus, who is called the Messiah.[h]
Explicitly says jesus is son of david, the jews understood this as well (Matthew 22:41–42)
41 Now while the Pharisees were gathered together, Jesus asked them this question: 42 “What do you think of the Messiah?[a] Whose son is he?” They said to him, “The son of David.”
I think we can conclude from the bible that jesus is son of david, even if joseph's sperm wasn't the one that made jesus, since marry is of david from marriage.
edit: Should be noted jesus would be son of joseph under law as well, meaning he is indeed of david.