I do not find any mention of a miraculous event regarding the pregnancy surrounding Mary's birth in the Bible. (as RC theology presumes was the case)
I do find in the Bible a miraculous ocurrance regarding Jesus' birth (as Protestant theology maintains). The miracle of the incarnation guarantees the sinlessnes of Jesus without constructing a doctrine of Mary's prior immaculate conception.
I think you're also being a little free with what you term protestant theology. You consider yourself the arbiter of what is protestant theology? Luther, Calvin and Zwingli are usually considered protestants and they disagree with you. Do you consider them Catholics?