In your case ignorance. The revolutionaries Luther, Calvin and Zwingli all believed in the perpetual virginity of Mary. Why don't you "learned" protestants ever mention that.
You can't expect all the heresy that Luther obtained as a Catholic to disappear overnight. That's alot of catholicism to beat out of someone in one lifetime.
As for the others Prots can be deceived. Scott Hahn would be one contemporary Prot that would come to mind..