So, your contention is that Luther and Calvin gave sermons on things they didn't actually believe? What other things did they speak about that they didn't actually believe? Isn't preaching a sermon on a topic a priority? I would think that if it was such a low priority, they would have remained silent on the matter. What about Wesley? He was preaching more than two centuries after Luther and Calvin?
Then again, if this is a "minor" thing that you disagree with the Reformers on, then why is it such a major issue to you today?
There are only so many hours in the day. There were larger issues for Luther and Calvin than researching and talking about the perpetual virginity of Mary. Of course, had they known that the worship veneration of Mary would developed to where it is today, perhaps they would have spent more time on it.