How do Protestants explain Zechariah 12:10-14, used by Roman Catholics to prove Mary's perpetual virginity?

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**.”