One thing that comes to my mind about this point is when Jesus on the cross said to John the beloved to take St Mary as his Mother. This stands out to me that St Mary had only one Son that died on the cross.

25 Now there stood by the cross of Jesus His mother, and His mother’s sister, Mary the wife of Clopas, and Mary Magdalene. 26 When Jesus therefore saw His mother, and the disciple whom He loved standing by, He said to His mother, “Woman, behold your son!” 27 Then He said to the disciple, “Behold your mother!” And from that hour that disciple took her to his own home. John 19:25-27

I have a question similar to this issue, but very trivial compared to the perpetual virginity of St Mary. In the bible does the “Red Sea” really mean the Sea was Red? Cause I heard it was actually a “mistake” and it should be the “Reed Sea” or “Sea of Reeds”. http://en.wikipedia.org/wiki/Reed_Sea