I agree Luke is careful...No, Luke is careful to articulate a virginal conception and defend it against critics. He mentions not once but twice that Mary is a virgin in his introduction to her (1:27). Mary's bewildered response to the angel's pronouncement in which she underscores her own virginity (1:34) leads to the angel's claim that the agent of her conception will be the Holy Spirit (1:35).
But think about what you typed. When the angel is talking there is no doubt that Mary is a virgin (I am assuming Luke is right for the purposes of this discussion). As you say, it gets mentioned twice. But as you also say: "the agent of her conception will be the Holy Spirit". That is, she will conceive at some point in the future. That does not preclude her having sex and conceiving, albeit with the involvement of the Holy Spirit (and there are other examples, such as when Sarah conceived Isaac).
I am not sure where your get "with haste" from. More importantly, however, I can see nothing in the text to suggest Mary had conceived Jesus before the three months expired and she returned home.As soon as the angel departs Mary leaves Nazareth with haste and travels to the hill country of Judea where she spends three months with Elizabeth, who is herself in seclusion, and afterward returns to her house (1:24,38b-40,56). Mary is kept out of the company of her betrothed Joseph and men generally in the narrative during which time the miraculous conception is implied to take place (1:42-43). When next we hear of her, Mary is still betrothed to Joseph yet pregnant and traveling with him to Bethlehem where she gives birth to her firstborn son (2:5-7).
The text would seem to be consistent with an angel visiting the virgin Mary and telling her she would conceive, Mary then going to tell Elizabeth about it, and staying with Elizabeth three months, then returning home to Joseph. She then had sex with Joseph, conceiving Jesus with the help of the Holy Spirit. This, of course, fits with Luke tracing Jesus genealogy through Joseph.
The point about the Davidic line is that the messiah - the king of the Jews - had to be of David's seed to fulfil the promise made to David.Luke does trace Jesus' claim to the Davidic line and throne through Joseph (1:27,32;3:23), but this is based on Joseph being Mary's husband, not because he is the biological father of Jesus, which suggestion Luke rules out by introducing his genealogy with reference to Jesus as the supposed son of Joseph. Mary, for her part in Luke's narrative, is a relative of Elizabeth (1:36), who was a descendant of Aaron (1:5).
1 Sam 7:...“‘The Lord declares to you that the Lord himself will establish a house for you: 12 When your days are over and you rest with your ancestors, I will raise up your offspring to succeed you, your own flesh and blood, and I will establish his kingdom. 13 He is the one who will build a house for my Name, and I will establish the throne of his kingdom forever.
Jesus has to be a flesh and blood descendant of David, not merely as the supposed son of Joseph. This was vital to Jesus' claim to messiahship.
I suggest that what we see here is Luke trying to appeal to both Jewish Christians, who demanded a messiah descended from David via Joseph, and gentile Christians who wanted Jesus to be the product of a virgin birth. He has carefully worded the text to support both, while not excluding the possibility of the other.