What do Protestants say was the source for the doctrine of the perpetual virginity of Mary? Not counting Martin Luther, who appears to have believed in the doctrine of the Perpetual Virginity of Mary, 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).



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.
One of the defining tenets of Protestant Christianity is sola scriptura--that the Scriptures are the fundamental basis for all doctrine.  This is in contrast to extra-biblical teachings.  The idea is that if it were important enough for us to know, God would have told us in the Scriptures themselves and not have us rely on extra-biblical teachings.  Protestants see the potential for great danger when there is reliance on the teaching of men rather than on the unchanging Word of God.

To answer your question specifically, Protestants generally believe that this was a teaching that was introduced by the Catholic church centuries after the life of Jesus.  There is no apostolic authority behind it, as it appears nowhere in Scripture.  All we are told is that Mary was a virgin at Jesus' conception and remained so until His birth, as @ryanOptini also noted.


  Then Joseph being raised from sleep did as the angle of the Lord
  had bidden him, and took unto him his wife, and knew her not till she had brought forth 
  her firstborn son: and he called his name Jesus.  Matthew 1:24-25


Nowhere in Scripture does it indicate that Mary remained a virgin after the birth of Christ.  This would certainly have been noteworthy, since it would not have been a normal thing for a married woman (and man, for that matter) to not have a consummated marriage and experience the normal and sacred aspects of marriage.  Yet, the best biblical support is quite vague at best.  

It is also problematic to Protestants as to why it would have been necessary for Mary to remain a virgin--and for Joseph to remain celibate towards her as her husband.  Why is this level of intimacy denied both Mary and Joseph and what could possibly be wrong with it?

Now, it may have been the Mary began to be held up as a female example for women to follow.  Men could look to Jesus Himself or any of the apostles to see a male example to follow.  So, perhaps it was well-intentioned.  God found favor with Mary, much like He had done with Noah in the Old Testament.  Her virginity as an unmarried woman is commendable as well.  And perhaps over time, Mary's virginity was turned into eternal virginity to really set her apart and give her even more honor.

Summary

So, again, it may have been well-intentioned, but Protestants generally believe it was a teaching introduced by the Catholic church long after the apostles were gone.  So, the teaching lacks apostolic authority as well as biblical support, which is extremely important if something so unexpected is to be believed and held as such an important doctrine.