So, we have two denominations shouting that MARY HAD NO SEX EVER.
I don't know if Orthodoxy is the other church (RCC and EO are churches, not denominations) you are referring to here, but we believe in it, yet it is not a matter of high importance. Or perhaps the other denomination is Lutherism. Luther believed in it wholeheartedly, but today's Lutherans not so much. You'll have explain this one to me.
So, we're waiting: "PROVE IT!!!!!!!!!!!!!!!!!! And I don't mean just your interpretation of something or some implication you see, I mean PROVE IT!!!!!!!!!!!!!!!!!!!!!
I'll do my best.
Let's start will the more obvious one first. Mary did not have other children. Language changes with time. Some want to apply 20th century usages to ancient language. Big problems can ensue when this happens: The term "brothers" and "sisters" in Hebrew usage at the time did not mean they had the same biological parents. This is not surprising since ancient Israel was a far more communal society than today's post-Reformation individualistic culture.
We have Jesus in John turning the care of His mother to John the Apostle. If Jesus
had these younger brothers and sisters, there would be no reason to do this--especially in that society. So, these younger siblings of Christ either all died by the time their older brother died or they didn't exist.
Even stronger is the historical argument. When did this perpetual virginity argument
begin? The medieval times? Reformation times? No. The first mention of it is in Origin's writings in 240. St. Athanasius in 350 uses the term "ever-virgin Mary." Now, if Mary had other children in 200 years there would be memory of them. If multiple siblings of Jesus were running around, they would have gotten married and have kids that people in 240 knew in the flesh. To make such an audacious argument that Jesus' siblings had no kids would have be absurd and laughed at if descendants of his relatives were walking around.
Now, let's talk about intercourse (not really, but you know what I mean). How do we know that Mary did not have sex? The is biblical argument about the gates of Jerusalem not being broke through. Many others talked about this.
Once again, we have the historical argument: In 340, St Athanasius uses the term "ever-virgin Mary" and the way it is written is not like he is using it for the first time. It was in common use. The Nicene Creed uses the term Virgin Mary. I think ever-virgin was used in the Fifth Ecumenical Council. It was in widespread use in first 1000 years of the church and no one really fought over it. So, the church either erred for 1000 years despite the fact that relatives of Jesus were walking around or it is true and no one argued over it because it was accepted fact.
The final argument is a harder one for me to make--the theological argument. The first words man hears about God is to take off the shoes because this is a holy place. We don't think much of holy places in America but they did in Israel. Could there be a holier place than where Jesus dwelled for nine months? There is nothing wrong with shoes, but in the wrong place (sacred land) it is inappropriate to where them. Even more so with intercourse. It is a beautiful thing, but only if done at the right time and in an appropriate manner.
This may not be enough evidence for you (although it is for most of the world's Christians). Evidence of ancient times is limited compared to today. I can prove where I was born, but that is only due to modern recordingkeeping. How do I know they didn't type the wrong date on my birth certificate? I can't know for sure, but we have to believe what others say at some point.
1) Mary actually married Joseph
2) The New Testament consists of the 27 books we have in it today. These books are inspired and have equal or more authority than the Old Testament.
3) The resurrection happened.
There is evidence of two and three (not so much of one). There is not irrefutable evidence. They are matters of faith.
Hope this helps.