Not counting Martin Luther, [who appears to have believed in the doctrine of the Perpetual Virginity of Mary](https://christianity.stackexchange.com/a/347/4), what do other Protestants think is the source and reason for Catholic adherence to the notion that the Blessed Virgin Mary was a virgin before giving birth, after giving birth and until the end of her earthly life? 

My assumption, as a Catholic, is that once the reformation took off, and people were interpreting the Bible for themselves, that they looked at the brothers and sisters of Christ mentioned in Mark and threw out the doctrine. 

They probably thought that it leads to 

  - worshiping Mary.
  - is an extra burden put on them by the Church.
  - is borne out of ignorance of scripture.
  - is a made up tradition.

But all these are my assumptions, what are the protestant assumptions about the source and reason for the existence of the doctrine of the perpetual virginity of Mary (the one that they absolutely do not adhere to).

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By perpetual virginity, I mean mainly that she remained a virgin during and after giving birth to Jesus.  

If the answer is purely biblically based is should at least acknowledge the fact that pre-reformation Christians were capable of reading and interpreting scripture.  I'd like to know what the reformers used as their arguments that most Protestants just take for granted nowadays. Unless, of course, they seriously weren't all that interested in convincing anyone who was not ignorant of scripture.