St. Jerome said:
Helvidius is at much superfluous trouble to make this word know refer to carnal knowledge rather than to acquaintance, as though any had ever denied that; or as if the follies to which he replies had ever occurred to any person of common understanding. He then goes on to say, that the adverb, until, denotes a fixed time when that should take place, which had not taken place before; so that here from the words, He knew her not until she had brought forth her first-born Son, it is clear, he says, that after that he did know her. And in proof of this he heaps together many instances from Scripture.
To all this we answer, that the word until is to be understood in two senses in Scripture. And concerning the expression, knew her not, he has himself shewn, that it must be referred to carnal knowledge, none doubting that it is often used of acquaintance, as in that, The child Jesus tarried behind in Jerusalem, and His parents knew not of it. [Luke 2:43]
In like manner, until often denotes in Scripture, as he has shewn, a fixed period, but often also an infinite time, as in that, Even to your old age I am He. [Isa 46:4] Will God then cease to be when they are grown old? Also the Saviour in the Gospel, Lo, I am with you always, even to the end of this world. [Matt 28:20] Will He then leave His disciples at the end of the world? Again, the Apostle says, He must reign till He has put His enemies under His feet. [1 Cor 15:25]
Be it understood then, that which if it had not been written might have been 58 doubted, is expressly declared to us; other things are left to our own understanding.
[ed. note: In other words, till, need not imply a termination at a certain point of time, but may be giving us information up to a point from which onwards there is already no doubt. Supposing an Evangelist thought the very notion shocking that Joseph should have considered the Blessed Virgin as his wife after he was a witness of her bearing God the Son, he would only say that the vision had its effect upon him up to that time when it was no longer necessary. Just as if, in speaking of a man like Augustine, one said, that, in consequence of some awful occurrence, he was in the habit of saying prayers till the time of his conversion, no one would suppose that he left them off on being converted.]
So here the Evangelist informs us, in that wherein there might have been room for error, that she was not known by her husband until the birth of her Son, that we might thence infer that much less was she known afterwards.