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.
Your explanation makes sense, and it is a legitimate one.
The point is not that sex within marriage is dirty or wrong. This is not the motivation for our belief that the Theotokos was a life-long virgin.
The reason we have the tradition is that it is the account passed down within the Church from the earliest times (this is in part what a tradition is.)
If you have a family story where your father tells you that his father told him that your grandfather told him that your great-grandfather did X -- then you will in turn tell your son the same thing, because you believe that your father didn't lie to you, nor his to him, etc...
Even if someone later comes along and tells your son that this story doesn't seem plausible, and that something else probably happened, you will stick with your family story unless there is hard proof that the other idea was true.
This is our "family story" passed down from the earliest times, and there is no convincing evidence to believe otherwise. And turn it from a "family story" to a tradition that is passed on within the living Body of Christ, and you will see why we really don't consider believing otherwise.
I certainly can understand why Protestantism would choose a different explanation. Since you believe that the only evidence that is valid from the 1st c. is what is found in the New Testament, and reject all histories other than the NT (at least all other Christian histories), then based only on the NT, your beliefs are reasonable..