Whether or not it is true, it is still highly implausible. Here is the situation, according to this story. Joseph had been married, had children (which were later called the brothers and sisters of Jesus even though there was no blood relationship whatsoever), was widowed and now was an elderly and impotent man. Mary, who was totally desperate to get married and could not attract any other man, had to settle for a man the age of her grandfather. She knew that this old geezer was impotent and that she would never be a mother. However, that was okey-dokey with her because she had a hang-up about marital sexual relations and had decided that not only did she not want any children, but she recoiled at the very thought of intercourse. She had determined that this sort of relationship was disgusting, if not outright sinful.
So, Joseph, seeing a prime opportunity, proposed to Mary. His children and other relatives thought he had lost his mind and could not believe that Mary would have the slightest interest in him. In the meantime, Mary's family thought she had lost her mind in wanting to marry a man who was older than her grandfather. But, as they say, love is blind.
All is well until Mary is approached by the angel who informs her that, despite her repulsion of all things sexual, she would get pregnant. She asked how this could be since she was a virgin and had not been sleeping around. The angel explained the process to her and she was fine with it. However, when Joseph found out about it he was ready to dump her, not being willing to understand that she actually might want children even though she had told him of her peculiar abhorence of marriage. However, an angel appeared to him in a dream and told him that all was well and he would have his virgin bride forever.
So, when the time came for Mary to give birth a miracle occured which permitted Mary to retain her hymen intact, and thus her virginity. Jesus Christ did not pass through her birth canal, but emerged from her side.
Now, if you believe this to be true, I have a really wonderful bridge in Brooklyn for sale.