St Augustine believed in the perpetual virginity of the Mother of God.
Moreover, when that burial is made an object of belief, there enters also the recollection of the new tomb, which was meant to present a testimony to Him in His destiny to rise again to newness of life, even as the Virgin’s womb did the same to Him in His appointment to be born. For just as in that sepulchre no other dead person was buried, whether before or after Him; so neither in that womb, whether before or after, was anything mortal conceived. [Of Faith and the Creed, 11]
This isn’t exactly demonstrative of the fact that he was in no sense a Protestant, since (if I remember correctly) some of the Reformers accepted her Perpetual Virginity. But at least we can say that he cannot be grouped with the vast majority of Protestants today on this score.
I was reading a Scott Hahn book recently, and in it he makes a salient observation about the fact that Mary clearly must have taken a vow of virginity. When she asked Gabriel how his word could be true, she said: “How shall this be done, because I know not man?” Hahn points out that she was no dummy. She knew about the birds and the bees, and she knew that she was engaged to Joseph. In short, the only way that her question makes any sense is if she had taken a vow of virginity. Otherwise the “how” was perfectly obvious, and her question would have been unnecessary: “He says I’m going to have a baby. Of course I am; I am getting married soon.”
No. That’s not what she said, because having babies was not part of the plan. St Augustine accepted this fact, and Protestants should too.