You don't have to be Catholic or Orthodox to believe this. Calvin and Luther also believed in perpetual virginity. Or at least considered it.
Extra-biblical dogma driving interpretation - the root of bad hermeneutics.
It's not necessarily extra-biblical. The scriptures teach us that the Temple of God is holy and that meticulous care was put into not profaning it. If such attention was given to mere bricks, how much more the temple who housed the Son of God? We are even given warning ourselves about our own bodies, as temples housing the Holy Spirit. But Mary is a unique case, not just in sharing the Spirit like every other Christian after Pentecost, but she housed the Incarnate Word himself for 9 months in the most concrete manner, in a way no one before or after will ever experience. Truly "God With Us" is most realized in Mary. The very idea of "the human body being the Temple of God" is most fully realized in her life. She is the firstfruits of what we all aspire to.
So if you put it in that light, it's natural to assume she wanted to keep her body sanctified after such a momentous act. That she couldn't simply go on living as a normal human being, acting like nothing happened, treating literally GOD IN HER WOMB like some forgettable conversation or casual walk down the street. The only kind of person who quickly dismisses this hasn't thought it through, in my humble opinion. Perhaps they might still come to the same conclusion after much thoughtfulness that perhaps she wasn't a perpetual virgin, but I'm a little wary of stances that are so decisive about it. They remind me more of how atheists behave, who don't believe in anything in the first place, and just revolt at all things God or spiritual related and like tearing things down for fun. This is why precisely why Reformers like Luther and Calvin were so grieved by the Radical Reformers of their day. Some didn't merely want to reform, but wanted to destroy everything wholesale if it even had a hint of being related to tradition. Sadly, it got so bad that Lutherans and Anabaptists even went to war and started killing each other eventually. That's how different Reformed thought is to Radical Reformed thought. The former were actually closer to Catholic, despite insisting on the Scriptures and good hermeneutics.
Secondly, if it is mere tradition, it should be said that it's a very, very, very old one. Perhaps you may still want to destroy old traditions all the same, but you should ask yourself why they even arose in the first place. This teaching was already spreading in the Protoevangelium of James, which is purported to be written circa 2nd century/145 BC (before Origen, who later commented on it). It had some strange teachings, but the fact that it was even circulating less than 100 years after the Apostles is remarkable. That's the same amount of time as WW1 is to us. Which isn't much. Some Church Fathers even balked at some of the contents (and even medieval ones, like Aquinas), but they didn't balk at the parts about Mary, funnily. It seemed that that part was an uncontroversial teaching already by the time. They balked at the other outlandish elements of the story.
