Just so I understand at least some of the Protestant positions on this matter, I am assuming that at least the followers of Luther and Calvin accept the canons of the Fifth Ecumenical Council and particularly:
"If anyone shall not confess that the Word of God has two nativities, the one from all eternity of the Father, without time and without body; the other in these last days, coming down from heaven and being made flesh of the holy and glorious Mary, Mother of God and always a virgin, and born of her: let him be anathema." Canon II
"IF anyone shall not call in a true acceptation, but only in a false acceptation, the holy, glorious, and ever-virgin Mary, the Mother of God, or shall call her so only in a relative sense, believing that she bare only a simple man and that God the word was not incarnate of her, but that the incarnation of God the Word resulted only from the fact that he united himself to that man who was born [of her];(1) if he shall calumniate the Holy Synod of Chalcedon as though it had asserted the Virgin to be Mother of God according to the impious sense of Theodore; or if anyone shall call her the mother of a man (Anthropotokon) or the Mother of Christ (Xristotokon), as if Christ were not God, and shall not confess that she is exactly and truly the Mother of God, because that God the Word who before all ages was begotten of the Father was in these last days made flesh and born of her, and if anyone shall not confess that in this sense the holy Synod of Chalcedon acknowledged her to be the Mother of God: let him be anathema." Canon VI
Now, it seems pretty clear that those who deny the perpetual virginity of the Most Holy Theotokos are anthemized by The Church and have been since the 500s.
Why don't they accept what Luther and Calvin actually said about the Holy Mother?
http://ic.net/~erasmus/RAZ460.HTM
I suppose I haven't studied all the canons of that Council "enough" :) to know if I agree with all of them or not. (Actually, I could only find summaries of the Council, but not a list of the Canons.) At any rate, I don't see any problems with Canon II.
Now, it seems pretty clear that those who deny the perpetual virginity of the Most Holy Theotokos are anthemized by The Church and have been since the 500s.
In Canon VI, unless it is included in one of the references I wasn't familiar with, it looked like Mary's perpetual virginity was sort of mentioned only in passing. It looked to me like the meat of the Canon was fine. However, if even that one part would constitute disagreeing with the Canon, then I suppose I would. If Mary was not a deity herself, then I guess I don't understand how she is diminished by the idea that she was a loving wife to her husband and a loving mother to other children.