One must remember as well, that Luther arrived on the scene at a time where there had been CENTURIES of darkness in the Christian world with only flickers of light here and there. The works of those flickers of light had been systematically destroyed by the RCC. So, just as a baby born in a womb, Luther had the evidence of what he had just been through, still upon him. That he came as far as he did in his understanding is testimony to God's work in his life.
Whether one believes Mary was a perpetual virgin is a secondary matter of doctrine.
God made sure that Luther got it right on the essentials.
What is clear is that Luther believed Mary was a mere woman. A blessed one. But clearly distinct from her creator.
What has become of Marian studies today is a Marian devotion that in itself is idolatrous. Attributes of God that never occurred to any of the early church writers (nor Luther) have been ascribed to her, and a term like Theotokos has become laden with the baggage of the Marian "veneration" which now exists.
So, what should Protestants do? We can recognize that Luther wasn't perfect and clearly missed the mark on some non-essentials. We can also go back to the spirit with which Luther wrote during most of his life - that is back to the Scripture. We should ascribe no more to Mary than what Scripture ascribes to her. That she was the mother of the person who was the 2nd member of the trinity is biblical. That he preceded her is also biblical. Calling her "Mary, Mother of Jesus" is scriptural. Then one's Christology can be truly Christocentric rather than getting wrapped up in the modern-day cult of Mary.