"That seems clear to me. Does that mean that the Orthodox do not believe in Mary's perpetual virginity?"
The Orthodox Church does believe in the perpetual virginity of the Theotokos. It is a very clear tradition of the Church, with evidence for its belief going way back. Most of these things about the Theotokos we consider to be part of the "inner tradition" of the Church.
By that we mean that they are not parts of the "kerygma," the things one preach or proclaim as the Good News in order to bring people to faith in Christ. They are a precious heritage, and they deepen our understanding of how we should live and spiritually grow, since the Theotokos is the prime exemplar for us of the fullness of being a Christian.
Regarding the verse in Matthew 1, the same word translated as "until" in that verse is the one translated as "unto" in the verse, "Lo, I am with you always, even unto the end of the world." We of course do not believe that God will be with us until the end of the world, and then will suddenly change course and abandon us.
That said, the word does not at all exclude the possibility of the Theotokos having carnal relations after the birth of Christ -- it is not evidence either way. So Protestants are correct in stating that there is no Scriptural evidence that proves her being ever-virgin.
I do find it interesting, though, that St. Matthew would take the time to point out that fact and say it in that way. It was already clear from vs 18 that Joseph could not have been the father of the child. I'd be interested in the Protestant explanation for why this verse existed or why they think that Joseph wouldn't have 'known' her...
Given that we Orthodox feel that Joseph would not have "walked on holy ground" where God was or had been, it makes sense that St. Matthew would have said something like this to indicate the reverence with which Joseph treated what was going on. We simply believe that that same reverence continued after her giving birth. Joseph being an elderly widower (in our tradition) simply makes it more clear.
But what is your analysis if you look to the point of each statement? It seems to me that in the Matt. 1 statement, the point was to be clear that Joseph did not "know" her before or during her pregnancy. But then after she gave birth, they lived as husband and wife. OTOH, the verse at the end of Matthew seems to emphasize that He will be with His children always during the time of "the world". The implication that He will continue to be "with us" (for us) after this time period is obvious. There is no such obvious implication with Mary to remain celibate, after she gave birth. In fact, isn't the implication just the opposite? To this day I do not understand the wish for Mary to not have been a wife to her husband, as if sex within marriage was dirty or wrong.
I do find it interesting, though, that St. Matthew would take the time to point out that fact and say it in that way. It was already clear from vs 18 that Joseph could not have been the father of the child. I'd be interested in the Protestant explanation for why this verse existed or why they think that Joseph wouldn't have 'known' her...
I don't know what the official Protestant view is, but I'd be happy to give you my two cents. :) To me, verse 18 establishes the virgin birth, along with verse 25. One important point is that even in verse 18, they were not yet actually married. Not even in verse 19, despite words like "husband" and "divorce". They were still in a state of betrothal. My understanding is that betrothal was a far more serious thing to the people of that time than it is to us today. No one backed out of a betrothal, unless of course the woman showed up pregnant by another man! :) It was a de facto marriage, but without the benefits. :)
This would explain why he did not "know" her. He was not married to her until after the angel had appeared to him and explained the situation. I take verse 24 as the first sign of actual marriage. Verse 25 would not make sense if they were not married at that time.