There are no verses or passages that conclude without doubt that Mary "remained" Virgin. I don't think anyone would dispute this.
There are a lot of things that do point to her remaining Virgin but those are found outside the bounds of the text itself. Did pious Jews abstain from relations after God did something miraculous or revealed Himself through some sign or wonder or miracle? Yes. Is it unlikely that Jesus had he had siblings would give Mary to St. John, the beloved when he even said to obey the Pharisees and did all that could be done in fulfilling all righteousness? Yes. It's unlikely. The fact that there are many many Old Testament verses that point to Mary as being a type of "ark" and "new eve" point to her ever Virginity (brought forth the firstborn)? Yes. Would a pious Jew dream of even going near something so Holy as the womb of Mary that contained the uncontainable God? No. A pious gentile wouldn't even dream of it.
But all of the above is outside the text and unfortunately can't, from a "sola scriptura" frame of reference rely on any of these realities to form a doctrine or teaching or even an understanding according to the "whole" of Protestantism. So it just becomes like most things in their tradition. Novelty. A nice thing to believe or not. A bad thing to believe or not. This is why these topics are just going to wind up being talked over the other side on both sides.
A Catholic who sees Holy Tradition as being authoritative can with authority assert the teachings of the church and sleep at night. However, the protestant allows himself only to make doctrines, teachings and dogmas according to their own understanding so long as it is based on (even if it's loosely based on) their reading of scripture. It's a personal and private matter and the only authority is in their personal self, granting what they believe independantly, as being "sound doctrine".
Does it matter whatsoever to Protestants that all the reformers believed in Mary's perpetual Virginity? No. Is there a solid paper trail of doctrine or teaching on Mary's ever-Virginity prior to the time when people outside the Church began to teach against it? No. Do we have a nice paper trail of the doctrine of the Holy Trinity as we express it today prior to groups and individuals teaching against it? No.
Dogma on either side isn't even an issue. A protestant sees salvation as being a private matter and so the protestant expression of "church" is private. They are accountable only to their interpretation of scripture alone and whatever other standards they personally or as a group set up for themselves. Even if it conflicts, contradicts and has no continuity with their fellow protestant.
So if the fathers, the very founders of their churches and church structure believe she was ever virgin it just doesn't matter to them. They, their own pastors, founders, leaders etc, according to the protestant mind, have no authority because the only authority is in their private choice to declare what they personally believe the Holy Spirit or hermeneutic approach has convinced them is the true stance to take. Their lack of tradition is their tradition. This is why if Luther came back to the church that bares his name he would protest against it and want to reform it.
Protestantism doesn't look much like itself if you even go back fifty years. How is it that you have nearly all the reformers and all the Christians world wide believing in her ever Virginity and then a mere hundred years later, not? A protestant today might think to themselves that those people from back in the day must not have been able to read the bible. But Luther translated the bible (even the books that are sometimes referred to as Apocrypha) from the original languages. Maybe Luther just didn't notice the "brothers and sisters" parts.
The fact is this. It boils down to Holy Tradition informing us of the scriptures and often the other way around. Some protestants today do recognize bits and pieces of Holy Tradition and have no trouble proclaiming them as truth. Truths such as the martyrdom of most of the Apostles. Why this matters? We are told to remember our leaders, to consider the outcome of their way of life and to imitate their faith. It's what the Church does, "according to the whole" but in Protestantism, they recollect as far as the text allows them and so there is no past, only the one they construct by reading the Holy Scriptures. Now does it matter to a Protestant that they all come to different conclusions on what happened in the past and what the Apostles taught and how the early Christians and the early Church is depicted in the scriptures? No.
Does it matter that Jesus is fully human and that there is an actual human that gave her flesh to him that took on flesh and so united God's nature to His creation? Does it matter that what Mary gave Christ is not unlike raising jacobs ladder to the heavens? That Jesus probably looked like his mother as all kids look like their parents? Does it matter that the very blood that we (I hope) consider precious was formed in the womb of the blessed Virgin? Does it matter that Mary is the first Christian? The first to have Christ "in" her as we are all to have Christ "in" us? Why does St. John see a typology between Mary, Israel, the Ark, the Church and interplays on them throughout Revelation and the gospel of St. John? Does it matter?
The way we treat these subjects are often poor. We think we have to be convinced or argued into belief instead of looking at who Christ is and seeking Christ in Mary. We reduce Mary to an easy bake oven that popped out a golden ticket so we can go to some Willy Wonka Chocolate Factory (heaven) instead of looking at God work through the very life of a person who gave birth to life himself. Who offered the fruit of her womb to humanity. We're quick to complain about Eve and give her, her credit and remember her work in the garden by handing humanity the fruit of the advesary's "word", that resulted in lowering man to the depths of Sheol, but when Mary hands humanity the fruit of the "word" of the Father, that humanity and all of creation might be raised, she's a "tool" God used to just get us to heaven. An oven. He "used" her...
Mary might not "save" you but she's a precious member of the Church and to remove her and not remember her and reduce her is to reduce Christ and take away from what he gave to her to give to us with thanksgiving (eucharist). She typifies the Christian life.
Her virginity goes beyond mere sexual whateverness. It has to do with being Holy as He is Holy. It's not a demonization of sex or marriage but a life of one lived as a perfect example of sex and marriage. How many of us can say that we gave birth to God? How many of us can say that we are so "highly favored"?
She matters. What we say about her matters.
Remember your leaders, who spoke the word of God to you. Consider the outcome of their way of life and imitate their faith.
Mary's life spoke volumes that were they written down, the world itself would not contain the books that would be written. Her life was Christ in the most intimate unimaginable way ever known by any creature. When on part suffers, the whole body suffers. When one part rejoices, the whole rejoices with her.