As I said, it's an idiom. I strongly suggest reading Blessed Jerome's tract.
OK, done that. I think you've suggested this a previous time. It was familiar.
One of the arguments Jerome put forward was, "
But when he continues, the Evangelist would never have applied the words, before they came together to persons who were not to come together, any more than one says, before he dined, when the man is not going to dine, I know not whether to grieve or laugh. Shall I convict him of ignorance, or accuse him of rashness? Just as if, supposing a person to say, Before dining in harbour I sailed to Africa, his words could not hold good unless he were compelled some day to dine in harbour. If I choose to say, the apostle Paul before he went to Spain was put in fetters at Rome, or (as I certainly might) Helvidius, before he repented, was cut off by death, must Paul on being released at once go to Spain, or must Helvidius repent after death, although the Scripture says In sheol who shall give you thanks?"
We do need to be careful, all of us including me, when we are comparing Greek constructions (and Hebrew) and English. However, there does seem to be a significant difference between Jerome's argument and what is said in Mt 1:25. Matthew says that Joseph did
not know Mary until she bore a son. None of Jerome's examples have that negative (that I could see). Thus there is a significant difference between:
1)
Before dining in harbour I sailed to Africa.
2) I did
not dine in harbour
until I sailed to Africa.
Better would be
a)
Before eating the fish and chips, I set sail for Africa.
b) I did
not eat the fish and chips
until I set sail for Africa.
As Jerome argues a) does not clearly tell us whether I ever ate the fish and chips, but b), with its negative, does indeed imply that I did. Mt 1:25 has that negative in it.
That is one area, at least, where Jerome's argument falls down, at least in English (as does the Psalm 110 comparison).
It might indeed be that Mary remained a virgin, but Mt 1:25 seems to say that she did not. If it is important to us whether she did, then I would have thought that the NT-writers would have made more of it and said it.
Might be wrong though