Mt. 1:25 – “but knew her not until she had borne a son; and he called his name Jesus”.
To say this sentence and the context that the sentence was written in, does not mean what it simply means, is to render the entire sentence worthless and unnecessary to Scripture.
You have a problem with “until” not meaning “until after Jesus was born”, and “firstborn” not meaning “first of more than one”, let’s focus on the word “KNEW” then.
This word, in the context of sex, or marriage, or having children, has always been about sexual intercourse. The sentence opens discussing the fact Joseph, who did not follow Jewish tradition of consummating - and thereby completing the validation of the marriage - with Mary, because she was with child form the Holy Spirit. This act still was not yet performed and it was a step that was normally done the night of the wedding. The two had not yet become one flesh, going back to Genesis. Joseph had not yet KNOWN Mary.
This statement says he DID wind up ‘knowing’ Mary, his wife, and consummating the marriage according to the Jewish traditions. A marriage wasn’t complete until that act was done.
Now one can argue that simply Joseph KNOWING Mary doesn’t mean she had more children, and it is possible. But the fact is the statement recorded says he did KNOW her, and the very fact Joseph did, means Mary could not have been a virgin. Joseph did not know her before Jesus was born, but afterwards, he did. Whether or not you believe Jesus had brothers or sisters.
To deny the clear meaning of the sentence and its context is to turn the sentence into something non-sensical and devoid of meaning, which begs the question if it doesn’t mean what it appears to mean, and in fact according to some, supposedly proves the opposite of what the clear meaning is, why is it there at all?
Even if you give the Roman Church their machination arguments for “until” and “firstborn”, the word “KNEW” in the context of sex and children, is about sexual intercourse, and this particular case, between Joseph and Mary. It means she wasn’t a virgin AFTER Christ was born, regardless of whether He had brothers and sisters.
I believe that Mary was a Godly woman and had sex with Joseph after Jesus' birth. But the verse you quote is a translation. The original language for "until" has a very specific sequence which means up until and has no linguistic value for the future. Looking at all of the evidence in the Bible, taking into account the original language, the Bible does not say either way if Mary was ever virgin or not.
Without tradition, there is no meaningful salvation difference between ever virgin and not. Logically, I would relate a virgin wife to a fig tree that doesn't bear fruit. It's just wrong.
Why would Mary ask the Angel how the birth of Jesus was to come about because she did not “know†man? Obviously, she knew the biological part of having a child and she was already espoused to Joseph. Even if she didn’t know the actual biological part of the equation every child or young adult knows that children result from marriage. This question/conversation only makes sense unless Mary was a virgin and planning on continuing to be one.