>>529632
Reply to Objection 1. As Jerome says (Contra Helvid. i): "Although this particle 'before' often indicates a subsequent event, yet we must observe that it not infrequently points merely to some thing previously in the mind: nor is there need that what was in the mind take place eventually, since something may occur to prevent its happening. Thus if a man say: 'Before I dined in the port, I set sail,' we do not understand him to have dined in port after he set sail: but that his mind was set on dining in port." In like manner the evangelist says: "Before they came together" Mary "was found with child, of the Holy Ghost," not that they came together afterwards: but that, when it seemed that they would come together, this was forestalled through her conceiving by the Holy Ghost, the result being that afterwards they did not come together.
Reply to Objection 2. As Augustine says (De Nup. et Concup. i): "The Mother of God is called (Joseph's) wife from the first promise of her espousals, whom he had not known nor ever was to know by carnal intercourse." For, as Ambrose says on Luke 1:27: "The fact of her marriage is declared, not to insinuate the loss of virginity, but to witness to the reality of the union."
Reply to Objection 3. Some have said that this is not to be understood of carnal knowledge, but of acquaintance. Thus Chrysostom says [Opus Imperf. in Matth., Hom. 1: among the spurious works ascribed to Chrysostom] that "Joseph did not know her, until she gave birth, being unaware of her dignity: but after she had given birth, then did he know her. Because by reason of her child she surpassed the whole world in beauty and dignity: since she alone in the narrow abode of her womb received Him Whom the world cannot contain."
Others again refer this to knowledge by sight. For as, while Moses was speaking with God, his face was so bright "that the children of Israel could not steadfastly behold it"; so Mary, while being "overshadowed" by the brightness of the "power of the Most High," could not be gazed on by Joseph, until she gave birth. But afterwards she is acknowledged by Joseph, by looking on her face, not by lustful contact.
Jerome, however, grants that this is to be understood of knowledge by intercourse; but he observes that "before" or "until" has a twofold sense in Scripture. For sometimes it indicates a fixed time, as Galatians 3:19: The law "was set because of transgressions, until the seed should come, to whom He made the promise." On the other hand, it sometimes indicates an indefinite time, as in Psalm 122:2: "Our eyes are unto the Lord our God, until He have mercy on us"; from which it is not to be gathered that our eyes are turned from God as soon as His mercy has been obtained. In this sense those things are indicated "of which we might doubt if they had not been written down: while others are left out to be supplied by our understanding. Thus the evangelist says that the Mother of God was not known by her husband until she gave birth, that we may be given to understand that still less did he know her afterwards" (Adversus Helvid. v).
Reply to Objection 4. The Scriptures are wont to designate as the first-born, not only a child who is followed by others, but also the one that is born first. "Otherwise, if a child were not first-born unless followed by others, the first-fruits would not be due as long as there was no further produce" [Jerome, Adversus Helvid. x]: which is clearly false, since according to the law the first-fruits had to be redeemed within a month (Numbers 18:16).
Reply to Objection 5. Some, as Jerome says on Matthew 12:49-50, "suppose that the brethren of the Lord were Joseph's sons by another wife. But we understand the brethren of the Lord to be not sons of Joseph, but cousins of the Saviour, the sons of Mary, His Mother's sister." For "Scripture speaks of brethren in four senses; namely, those who are united by being of the same parents, of the same nation, of the same family, by common affection." Wherefore the brethren of the Lord are so called, not by birth, as being born of the same mother; but by relationship, as being blood-relations of His. But Joseph, as Jerome says (Contra Helvid. ix), is rather to be believed to have remained a virgin, "since he is not said to have had another wife," and "a holy man does not live otherwise than chastely."
Reply to Objection 6. Mary who is called "the mother of James and Joseph" is not to be taken for the Mother of our Lord, who is not wont to be named in the Gospels save under this designation of her dignity–"the Mother of Jesus." This Mary is to be taken for the wife of Alphaeus, whose son was James the less, known as the "brother of the Lord" (Galatians 1:19).
tl;dr Mother of God is ever-virgin. Deal with it.