The apologist Dave Armstrong has
a nice roundup of the doctrine of “virginitas in partu”, including physical virginity with an intact hymen.
And yes, this is an ancient doctrine. And yes, it is in fact at the level of dogma, although there is dispute over whether it is “de fide” or one step down, at “sententia certa” (ie, you have to believe it because it ties in closely to “de fide” stuff).
There has been a lot of squeamishness about women’s genitalia that has prevented frank religious instruction about this in our age, whereas people back in the day were very matter-of-fact about it.
Of course Mary’s hymen didn’t have to remain intact for her to remain a virgin sexually. But then, there didn’t have to be a Star, or singing angels, or any of the other signs. Mary’s continued physical virginity is a sign screaming LOOK! THIS IS IMPORTANT STUFF, just like white robes and crowns and the wounds on Christ’s hands and feet.
Heck, Christ would have been both God and man if He’d been born with a defective willy; but He wasn’t, and that was a sign too. And that’s why you see all those Renaissance paintings showing Baby Jesus with no diaper on; the artist is showing an important sign of the Incarnation of Jesus, the Second Adam. And Mary is shown breastfeeding for many prophetic and Scriptural reasons, but mostly to show that she was truly virgin and mother.
Oh, and “virginal integrity” means virginal wholeness-of-hymen, just like the physical integrity of an airplane means there aren’t any holes in it.
And St. Thomas Aquinas had questions about the Immaculate Conception of Mary (ie, by her mom, St. Ann), not about the Virgin Birth of Jesus Christ.
As for the question of birth canal or no birth canal, it doesn’t really matter and there has been no pronouncement either way. What counts is that the birth was miraculous, right down to how it physically happened, and the hymen didn’t get affected. Many theologians like seeing the birth canal and hymen as the closed arch and gate of Ezekiel’s prophecy (which is part of why Mary is called Porta Caeli). Others like the closer Adam analogy. Either way, Jesus passed through a closed door, and Mary felt no pain. There are probably a few other alternatives on the manner of birth; but it wasn’t like a normal birth, just like Jesus’ conception was not normal. If the conception doesn’t worry you, why would the birth?
There are plenty of holy matriarchs and women saints who have given birth in edifying normal ways, if we need role models.