For those Protestants that believe that Mary had other children after Jesus was born, what do they make of Zechariah 12:10-14?
10 “And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and supplication. They will look on me, the one they have pierced, and they will mourn for him as one mourns for an only child, and grieve bitterly for him as one grieves for a firstborn son. 11 On that day the weeping in Jerusalem will be as great as the weeping of Hadad Rimmon in the plain of Megiddo. 12 The land will mourn, each clan by itself, with their wives by themselves: the clan of the house of David and their wives, the clan of the house of Nathan and their wives, 13 the clan of the house of Levi and their wives, the clan of Shimei and their wives, 14 and all the rest of the clans and their wives.
All Christians agree that Jesus was pierced and was a firstborn son. Why should we not also conclude from this passage that Jesus was also an only child? One Catholic article makes this point: http://whynotcatholicism.net/view/mary-ever-virgin
To connect Mary with this passage, is added this from Luke 2:
34 Then Simeon blessed them and said to Mary, his mother: “This child is destined to cause the falling and rising of many in Israel, and to be a sign that will be spoken against, 35 so that the thoughts of many hearts will be revealed. And a sword will pierce your own soul too.”