Objection 1. It would seem that Christ should not have been born of an espoused virgin. For espousals are ordered to carnal intercourse. But our Lord's Mother never wished to have carnal intercourse with her husband; because this would be derogatory to the virginity of her mind. Therefore she should not have been espoused.
Objection 2. Further, that Christ was born of a virgin was miraculous, whence Augustine says (Ep. ad Volus. cxxxvii): "This same power of God brought forth the infant's limbs out of the virginal womb of His inviolate Mother, by which in the vigor of manhood He passed through the closed doors. If we are told why this happened, it will cease to be wonderful; if another instance be alleged, it will no longer be unique." But miracles that are wrought in confirmation of the Faith should be manifest. Since, therefore, by her Espousals this miracle would be less evident, it seems that it was unfitting that Christ should be born of an espoused virgin.
Objection 3. Further, the martyr Ignatius, as Jerome says on Matthew 1:18, gives as a reason of the espousals of the Mother of God, "that the manner of His Birth might be hidden from the devil, who would think Him to be begotten not of a virgin but of a wife." But this seems to be no reason at all. First, because by his natural cunning he knows whatever takes place in bodies. Secondly, because later on the demons, through many evident signs, knew Christ after a fashion: whence it is written (Mark 1:23-24): "A man with an unclean spirit... cried out, saying: What have we to do with Thee, Jesus of Nazareth? Art Thou come to destroy us? I know... Thou art the Holy one of God." Therefore it does not seem fitting that the Mother of God should have been espoused.
Objection 4. Further, Jerome gives as another reason, "lest the Mother of God should be stoned by the Jews as an adulteress." But this reason seems to have no weight, for if she were not espoused, she could not be condemned for adultery. Therefore it does not seem reasonable that Christ should be born of an espoused virgin.
On the contrary, It is written (Matthew 1:18): "When as His Mother Mary was espoused to Joseph": and (Luke 1:26-27): "The angel Gabriel was sent... to a virgin espoused to a man whose name was Joseph."
I answer that, It was fitting that Christ should be born of an espoused virgin; first, for His own sake; secondly, for His Mother's sake; thirdly, for our sake. For the sake of Christ Himself, for four reasons. First, lest He should be rejected by unbelievers as illegitimate: wherefore Ambrose says on Luke 1:26-27: "How could we blame Herod or the Jews if they seem to persecute one who was born of adultery?"
Secondly, in order that in the customary way His genealogy might be traced through the male line. Thus Ambrose says on Luke 3:23: "He Who came into the world, according to the custom of the world had to be enrolled Now for this purpose, it is the men that are required, because they represent the family in the senate and other courts. The custom of the Scriptures, too, shows that the ancestry of the men is always traced out."
Thirdly, for the safety of the new-born Child: lest the devil should plot serious hurt against Him. Hence Ignatius says that she was espoused "that the manner of His Birth might be hidden from the devil."
Fourthly, that He might be fostered by Joseph: who is therefore called His "father," as bread-winner.
It was also fitting for the sake of the Virgin. First, because thus she was rendered exempt from punishment; that is, "lest she should be stoned by the Jews as an adulteress," as Jerome says.
Secondly, that thus she might be safeguarded from ill fame. Whence Ambrose says on Luke 1:26-27: "She was espoused lest she be wounded by the ill-fame of violated virginity, in whom the pregnant womb would betoken corruption."
Thirdly, that, as Jerome says, Joseph might administer to her wants.
This was fitting, again, for our sake. First, because Joseph is thus a witness to Christ's being born of a virgin. Wherefore Ambrose says: "Her husband is the more trustworthy witness of her purity, in that he would deplore the dishonor, and avenge the disgrace, were it not that he acknowledged the mystery."
Secondly, because thereby the very words of the Virgin are rendered more credible by which she asserted her virginity. Thus Ambrose says: "Belief in Mary's words is strengthened, the motive for a lie is removed. If she had not been espoused when pregnant, she would seem to have wished to hide her sin by a lie: being espoused, she had no motive for lying, since a woman's pregnancy is the reward of marriage and gives grace to the nuptial bond." These two reasons add strength to our faith.
Thirdly, that all excuse be removed from those virgins who, through want of caution, fall into dishonor. Hence Ambrose says: "It was not becoming that virgins should expose themselves to evil report, and cover themselves with the excuse that the Mother of the Lord had also been oppressed by ill-fame."
Fourthly, because by this the universal Church is typified, which is a virgin and yet is espoused to one Man, Christ, as Augustine says (De Sanct. Virg. xii).
A fifth reason may be added: since the Mother of the Lord being both espoused and a virgin, both virginity and wedlock are honored in her person, in contradiction to those heretics who disparaged one or the other.
Reply to Objection 1. We must believe that the Blessed Virgin, Mother of God, desired, from an intimate inspiration of the Holy Ghost, to be espoused, being confident that by the help of God she would never come to have carnal intercourse: yet she left this to God's discretion. Wherefore she suffered nothing in detriment to her virginity.
Reply to Objection 2. As Ambrose says on Luke 1:26: "Our Lord preferred that men should doubt of His origin rather than of His Mother's purity. For he knew the delicacy of virgin modesty, and how easily the fair name of chastity is disparaged: nor did He choose that our faith in His Birth should be strengthened in detriment to His Mother." We must observe, however, that some miracles wrought by God are the direct object of faith; such are the miracles of the virginal Birth, the Resurrection of our Lord, and the Sacrament of the Altar. Wherefore our Lord wished these to be more hidden, that belief in them might have greater merit. Whereas other miracles are for the strengthening of faith: and these it behooves to be manifest.
Reply to Objection 3. As Augustine says (De Trin. ii.), the devil can do many things by his natural power which he is hindered by the Divine power from doing. Thus it may be that by his natural power the devil could know that the Mother of God knew not man, but was a virgin; yet was prevented by God from knowing the manner of the Divine Birth. That afterwards the devil after a fashion knew that He was the Son of God, makes no difficulty: because then the time had already come for Christ to make known His power against the devil, and to suffer persecution aroused by him. But during His infancy it behooved the malice of the devil to be withheld, lest he should persecute Him too severely: for Christ did not wish to suffer such things then, nor to make His power known, but to show Himself to be in all things like other infants. Hence Pope Leo (Serm. in Epiph. iv) says that "the Magi found the Child Jesus small in body, dependent on others, unable to speak, and in no way differing from the generality of human infants." Ambrose, however, expounding Luke 1:26, seems to understand this of the devil's members. For, after giving the above reason--namely, that the prince of the world might be deceived--he continues thus: "Yet still more did He deceive the princes of the world, since the evil disposition of the demons easily discovers even hidden things: but those who spend their lives in worldly vanities can have no acquaintance of Divine things."
Reply to Objection 4. The sentence of adulteresses according to the Law was that they should be stoned, not only if they were already espoused or married, but also if their maidenhood were still under the protection of the paternal roof, until the day when they enter the married state. Thus it is written (Deuteronomy 22:20-21): "If... virginity be not found in the damsel... the men of the city shall stone her to death, and she shall die; because she hath done a wicked thing in Israel, to play the whore in her father's house."
It may also be said, according to some writers, that the Blessed Virgin was of the family or kindred of Aaron, so that she was related to Elizabeth, as we are told (Luke 1:36). Now a virgin of the priestly tribe was condemned to death for whoredom; for we read (Leviticus 21:9): "If the daughter of a priest be taken in whoredom, and dishonor the name of her father, she shall be burnt with fire."
Lastly, some understand the passage of Jerome to refer to the throwing of stones by ill-fame.
Source: Summa Theologica (New Advent)