The perpetual virginity of Mary is a Catholic belief, not a Protestant one.
Not so; all the early Protestant leaders, including Martin Luther, John Calvin and Ulrich Zwingli believed that Mary remained a virgin after giving birth to Jesus:
Martin Luther: “It is an article of faith that Mary is Mother of the Lord and still a virgin…Christ, we believe, came forth from a womb left perfectly intact.” (Weimer, The Works of Luther, English Transl. by Pelikan, Concordia, St. Louis, v.11,pp. 319-320; v. 6 p. 510.)
"Christ…was the only Son of Mary, and the Virgin Mary bore no children besides Him…“brothers” really means “cousins” here, for Holy Writ and the Jews always call cousins brothers. (Sermons on John, chapters 1-4, 1537-39.) “He, Christ, our Savior, was the real and natural fruit of Mary’s virginal womb…This was without the cooperation of a man, and she remained a virgin after that.” (Ibid.)
“There have been certain folk who have wished to suggest that from this passage (Matt 1:25) that the Virgin Mary had other children than the Son of God, and that Joseph then dwelt with her later; but what folly this is! For the gospel writer did not wish to record what happened afterwards; he simply wished to make clear Joseph’s obedience and to show also that Joseph had been well and truly assured that it was God who had sent His angel to Mary. He had therefore never dwelt with her nor had he shared her company…And besides this Our Lord Jesus Christ is called the first-born. This is not because there was a second or a third, but because the gospel writer is paying regard to the precedence. Scripture speaks thus of naming the first-born whether or not there was any question of the second.” (Sermon on Matthew 1:22-25, published 1562.) Ulrich Zwingli:
“I esteem immensely the Mother of God, the ever chaste, immaculate Virgin Mary…Christ…was born of a most undefiled Virgin.” (Stakemeier, E. in De Mariologia et Oecumenismo, Balic, K., ed., Rome, 1962, p. 456.) “I firmly believe that Mary, according to the words of the gospel as a pure Virgin brought forth for us the Son of God and in childbirth and after childbirth forever remained a pure, intact Virgin.” (Zwingli Opera, Corpus Reformatorum, Berlin, 1905, in Evang. Luc., v. 1, p. 424.)
So even the Reformers knew and acknowledged that the perpetual virginity of Mary is a truth rooted in both Scripture and Church history:blessyou: