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.”
The sense of Zech 12:10 "only" is an expression of one of a kind.

Here is another use as regards Abraham and Isaac.


  And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of.


The word "only" is yachiyd.  It does not necessarily mean one and only with no other children, but as unique, special, only in the sense of the one child of promise.

So, Zech 12:10 is not about Mary and Joseph, but rather, all about Jesus Christ the only Son.