From Archbishop Averky: On The Birth Of Christ
"And knew her not till she had brought forth her firstborn son." "Till", "until,"-this does not mean that after the birth of Jesus he "knew" her and began to live with her as a wife. St. John Chrysostom rightly observes that it is incorrect to assume that such a righteous man as Joseph would decide to "know'' the holy Virgin after she had so miraculously become a mother, In this case the word "till." can in no way be understood in the same way as it is interpreted by Protestants and other sectarians-who have no veneration for the Mother of God. These latter choose to believe that until the birth of Christ, Joseph did not know the Holy Virgin, but afterwards he did know her. St. John Chrysostom states on the contrary, that Joseph never knew her. In the Holy Scripture this word is used for example, in the verse concerning the end of the Flood: a raven "went forth to and fro, till the waters were dried up from off t h e earth" (Gen. 8:6), but ever afterwards it did not return. Or, for example, the words of the Lord: "Lo, I am with you always, even till the end of the world" (Matt. 28:20); this, of course, does not mean, as the Blessed Theophilactus rightly observes, that after the end of the world Christ will no longer be with us. No! then all the more will He be with us.
Here Christ is called the "firstborn" likewise not because after Him the Holy Virgin had more children, but only because He was the first to be born and also the only one.
In the Old Testament God commands that every first-born male child is to be dedicated to Him, regardless of whether or not there shall be other children. If in the Gospel there is made mention of "the brothers of Jesus Christ" (Matt. 12:46; John 2:12; and others), this in no way means that He had brothers after the flesh. Tradition testifies that most likely these were the children of Joseph-the-Betrothed, from his first marriage.