I don't think this matter can be resolved by means of resorting to Jewish history. Virgin birth was not a concept there, but it was certainly a common literary structure among the Greeks and Romans to designate a heroic person, whether Alexander or Octavian. The authors of Matthew and Luke were obviously reaching out to a wider audience than the Jewish community, and they were willing to adopt the method that had been used by Greek writers. To the Greeks, virgin birth was only a literary metaphor, and to reintroduce this metaphor back into Jewish culture as historical fact and then argue the pro and cons within this foreign context only leads to confusion and a debate that has no real significance.
The Jewish Yahoshua sects did not advocate a virgin birth, but rather vehemently denied it. This fact, in itself, is reason enough to suppose that virgin birth was an innovation and not part of the earliest teachings. Literary historical fiction does not require that all aspects of the setting be entirely correct, for what is important is the message that the author hopes to support, I.e., his own agenda. If he chooses to use metaphorical language to advance this agenda, as was common in literature of his time, then we should not read it as though it has to fit the assumptions of later Christian writers of Catholic and Protestant traditions. Such a parochial view creates the argument where none was necessary.
The idea that Joseph and Mary were real people may or may not be true. We have no way of knowing that. However, the Joseph and Mary that we meet in the gospels are literary constructs, and the story of virgin birth should not be given any more credence than the fable of Olympias conceiving the baby Alexander via a thunder bolt shooting down from Zeus on Mount Olympus. Obviously, this Olympia, the mother of Alexander, is a fictional character, even though there was indeed a real mother of Alexander. The story itself is mythic, even though it is told against a historical backdrop. If we take the fable as historical fact, then we cease to be rational people. This is also true of the gospel narrative. Places and people mentioned may be factual, but the writing itself should not be confused with history, and if there is a conflict with history, then chalk it up to the ignorance of the writer or perhaps his need to incorporate bogus material to support his position. After all, that is often the case with literature used as propaganda. The emphasis is on message, not particulars. This fact is demonstrated by the many contradictions we find in the New Testament and the willingness of the Church to incorporate them into one volume considered holy writ.
Common sense tells us that no one was born of a virgin in late antiquity - not Alexander, nor Octavian, and certainly not someone called Jesus Christ. I have not been able to find any reliable historical reference to any such person, and it seems that he is much the same sort of "mystery cult" figure as Hiram Abif and Christian Rosencrans.
If I read the masonic rituals, I find a lot of information about Hiram that is historically impossible, but the message of masonry remains. I also notice that Hiram and Jesus share a lot of the same mythic tales, and therefore I have to assume that they are much the same kind of characters. Few masons with any understanding of the ceremonies would claim historicity for the events portrayed in the lodges, and although Hiram is placed in a very historical setting, the words he speaks and the deeds he performs are part of a mystery play, and not to be confused with history. If I find a flaw in the history of the life of Hiram, it would be needless to argue the point, because Hiram's historical presence is a metaphor whose meaning is revealed in initiation and not in any analysis of the past. This is also true of Jesus. The gospels, as the masonic rituals, tell of the "path" that one travels in spiritual ascent, and the Jesus character is a metaphor that reveals various steps on the way. Just as there are many contradictions in the masonic rituals concerning Hiram and history, there are the same kinds of contradictions in the New Testament. In both cases, history is obviously of little importance to the writers.
So, whatever "betrothal" meant to Jews, or whenever marriages were consummated by Jewish couples, or whether Jesus had 7 brothers and sisters or none, all such considerations is based on an understanding of these stories that has been filtered through Orthodox Christianity over 1,600 years and makes a supposition concerning history that was never inherent in the original intent of the Nazorean community that created Yahoshua, as masons created Hiram Abif.