"And knew her not",.... Or "but he knew her not", kai answering to the Hebrew w that is, had carnal knowledge of her, or copulation with her, though his wife. The words are an euphemism, or a modest way of expressing the conjugal act, and is a very ancient one, see Ge 4:1 and what has been used in nations and languages.
And this conduct of his was necessary,till she had brought forth her firstborn; that it might be manifest not only that she conceived, being a virgin, but also that she brought forth, being a virgin: for both are signified in the prophecy before related, "a virgin shall conceive and bring forth a son"; which is all one as if it had been said, a virgin shall conceive, and "a virgin" shall bring forth a son.
The "firstborn" is that which first opens the womb of its mother, whether any follows after or not, Ex 13:12. Christ is called Mary's firstborn, because she had none before him, whether she had any after him or not; for her perpetual virginity seems to be no necessary article of faith: for when it is said,Joseph knew her not till she had brought forth, the meaning is certain that he knew her not before.
But whether he afterwards did or not, is not so manifest, nor is it a matter of any great importance; the word "until" may be so understood as referring to the time preceding, that the contrary cannot be affirmed of the time following, 2Sa 6:23 and which may be the case here, and is indeed generally understood so; and it also may be considered as only expressive of the intermediate time, as in Mt 5:26 as Beza observes.
Christ was "her firstborn" as he was man, and the firstborn of God, or his first and only begotten, as the Son of God. It is further observed, that she "called his name Jesus", as was foretold to her, or ordered her by the Angel, Lu 1:31 and to Joseph, Mt 1:21.