One of the claims of the Gospel is that Mary was a virgin, not only in the conception of Jesus but also during the delivery.
It's sort of an odd thing to mention, or even to really care about. What difference could it make if Mary remained virgin through childbirth?
"The common assumption today is that early Christians fussed about such things because they were uncomfortable with the human body, and particularly touchy about sex. But that's anachronistic. In the Scriptures as here, material Creation is affirmed; the very idea of the Incarnation shows that a real flesh-and-blood body can bear the divine, as a candlewick bears flame."
Today this concept is taken for granted by a majority of people who call themselves Christians. I won't say all because I know that neo-Gnostic sects come up all the time. So 'a majority' it is. But back then it was a completely insane and new concept in a Jewish faith. God dwelling within a mortal form? But today we know that salvation does not just involve our souls but also our flesh. Do we not also look forward to a bodily resurrection?
The early Christians had a very high respect for the physical body. It's why they sought to gather the remains of martyrs, even at great danger to themselves. Until recently it was not allowed for Roman Catholics to be cremated because of the possibility that they were doing it out of a disregard for the body which God created. It's still (to my knowledge) discouraged heavily in the Orthodox Church. The point is that if the early Christians had been uncomfortable with the human body they would not have treated the remains with such veneration. If there was a disregard for it or a belief that the body was tainted or superfluous then it would have been ignored.
"After all, it is not the physical body that is the problem. Jesus taught that it is not what a person eats that defiles him, but rather what comes out of his mouth and heart. He explained that the damage inside a human being is integral, like a sickly tree that cannot bear good fruit. So sin is not so much a culpable deed as a self-inflicted wound."
Most ancient cultures tended to be less squeamish about the body and its functions than we are today. They lacked modern facilities, remember. Many of them lived in very small spaces with large families. Not that long ago my own great grandparents and their children lived in a one room house. They had six kids. Think about it for a second there. We shelter our children now from things that we don't think they should know, for fear that they will act on this knowledge before they're old enough. But in the past this wasn't an option and would have been viewed as odd.
In this Gospel there is frank discussion by a priest about what they're going to do when Mary is due to begin her periods. References to breast feeding are plenteous. At the end of her labor Mary says, bluntly: "That which is within me presses me to come forth." As opposed to a more fanciful recounting of the same scene in a work entitled 'The Ascension of Isaiah' where Mary, after two months of pregnancy suddenly sees a baby appear and then discovers she's no longer pregnant. Mary's post-birth virginity is even verified within the story in the only practical way it could have been - and there is no indication that any of this shocked the devout original hearers.
The fact is that there was really no good reason for the early Christians to claim a virgin birth for Christ. It wasn't something likely to win over Jews - they weren't expecting the Messiah to be born of a virgin. Nor were they expecting Him to die on a cross. The claims of the virgin birth opened the Christians up to derision and accusations of fabricating the story. In addition, admitting that St. Joseph, Mary's husband, was not the father of Jesus invited the obvious charge that He was 'born of fornication'. "There was no rhetorical advantage to claiming a virgin birth for Jesus, and the likeliest reason the early Christians stood by it so stoutly was that they believed it was true."
But why would Mary remain a virgin through the birth? As we said, there's no rhetorical advantage to any portion of this story. So why? Gregory of Nyssa suggests as a theory that the pain of childbirth is one of the consequences of the Fall, so when God acts to reverse the damage, every element becomes a joy. "Just as the bush Moses saw was filled with the blazing presence of God yet not destroyed, so Mary's bodily integrity was not altered by childbirth."
The Protoevangelion shows that Mary was a virgin up to, and through, the birth of Christ. It does not, however, comment on whether or not she remained a virgin her entire life. Her perpetual virginity was the consensus of the early church and was universally believed until recent centuries. Even some Reformation leaders, Zwingli and Luther among them, upheld Mary's perpetual virginity.
Some people have tried to argue that because of the verse: "Joseph knew her not until she had borne a son." that meant that Joseph obviously knew her after she had borne Jesus.
That argument just sort of fails. If we're sticking to Biblical examples, Michal in the Old Testament is said not to have borne any children until she died. Does that mean that she had children after her death? No.
Likewise, Jesus' status as first born does not necessarily mean that there were second, third, etc. born children. First born is simply the first child born to a couple or a person individually. I would still be my mother's first born even if she'd never had any other children.
Many consider the strongest evidence for Mary not having any other children the fact that Jesus consigned her care to St. John at the cross. If she had had any biological sons her care would have gone to them.
This, of course, is just extremely brief mentioning of different arguments. People write books and treatises on these subjects.