Why did the Blessed Virgin Mary marry?

The Gospels do state that Mary was a virgin at the time of the Annunciation, but they do not per se say that she had intended to remain a virgin all trough life, in spite of her betrothal to Joseph. In those days, unmarried girls were called virgins, as is evident from the Parable of Ten Virgins. 1 Cor 7:36 (KJV) reinforces the fact :

"But if any man think that he behaveth himself uncomely toward his virgin, if she pass the flower of her age, and need so require, let him do what he will, he sinneth not: let them marry." ... He is not sinning; they can get married."

The very words "before they came together " (Mtt 1:18) state in unequivocal terms that Joseph and Mary had, after their betrothal and before the Annunciation, intended to consummate their union by marriage. It was the Annunciation and the events thereafter that changed the course of their life.

Why did the Blessed Virgin Mary marry?

There are several reasons why the Blessed Virgin Mary married St. Joseph.

Let us start with the Catholic Encyclopedia.

Her betrothal to Joseph

The apocryphal writings to which we referred in the last paragraph state that Mary remained in the Temple after her presentation in order to be educated with other Jewish children. There she enjoyed ecstatic visions and daily visits of the holy angels.

When she was fourteen, the high priest wished to send her home for marriage. Mary reminded him of her vow of virginity, and in his embarrassment the high priest consulted the Lord. Then he called all the young men of the family of David, and promised Mary in marriage to him whose rod should sprout and become the resting place of the Holy Ghost in form of a dove. It was Joseph who was privileged in this extraordinary way.

Jewish maidens were considered marriageable at the age of twelve years and six months, though the actual age of the bride varied with circumstances. The marriage was preceded by the betrothal, after which the bride legally belonged to the bridegroom, though she did not live with him till about a year later, when the marriage used to be celebrated. All this agrees well with the language of the Evangelists. St. Luke (1:27) calls Mary "a virgin espoused to a man whose name was Joseph"; St. Matthew (1:18) says, "when as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost". As we know of no brother of Mary, we must suppose that she was an heiress, and was obliged by the law of Numbers 36:6 to marry a member of her tribe. The Law itself prohibited marriage within certain degrees of relationship, so that the marriage of even an heiress was left more or less to choice.

According to Jewish custom, the union between Joseph and Mary had to be arranged by the parents of St. Joseph. One might ask why Mary consented to her betrothal, though she was bound by her vow of virginity. As she had obeyed God's inspiration in making her vow, so she obeyed God's inspiration in becoming the affianced bride of Joseph. Besides, it would have been singular among the Jews to refuse betrothal or marriage; for all the Jewish maidens aspired after marriage as the accomplishment of a natural duty. Mary trusted the Divine guidance implicitly, and thus was certain that her vow would be kept even in her married state.

Another reason why the Virgin Mary married St. Joseph comes indirectly from Scripture. When Joseph found out Mary was with child, decided to put Mary away privately in order to spare Mary the shame of having a child out of wedlock. In taking Mary as his wife, the Holy Family was looked upon as any other Jewish family living in Palestine at that moment.

18 Now the generation of Christ was in this wise. When as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost. 19 Whereupon Joseph her husband, being a just man, and not willing publicly to expose her, was minded to put her away privately. 20 But while he thought on these things, behold the angel of the Lord appeared to him in his sleep, saying: Joseph, son of David, fear not to take unto thee Mary thy wife, for that which is conceived in her, is of the Holy Ghost. 21 And she shall bring forth a son: and thou shalt call his name Jesus. For he shall save his people from their sins. 22 Now all this was done that it might be fulfilled which the Lord spoke by the prophet, saying: 23 Behold a virgin shall be with child, and bring forth a son, and they shall call his name Emmanuel, which being interpreted is, God with us. 24 And Joseph rising up from sleep, did as the angel of the Lord had commanded him, and took unto him his wife. 25 And he knew her not till she brought forth her firstborn son: and he called his name Jesus. - Matthew 1: 18-25

Origen gives us yet another reason why Mary and Joseph married one another. Mary’s virginity was not noticed by Satan due to her marriage with St. Joseph.

To begin in the East, in his Homilies on the Gospel of Luke, Origen considers the perpetual virginity of Mary already a settled point of doctrine, fully consistent with the biblical testimony (see Homily 7.4). He also points out that the reason Mary was frightened by the greeting of the angel, “Hail, full of grace,” is that she, as a good student of the Law, knew that this form of address was unprecedented in Scripture (Homily 6.7). In other words, Origen is pointing out that Mary’s participation in grace is unique, and this conviction continues in both Eastern and Western Christianity, a grace of sinlessness in the East, and Immaculate Conception in the West. For Origen, however, the uniqueness of Mary is not isolated from her marriage to Joseph. His comments on her uniqueness come immediately after a discussion of the divine dispensation which committed the Incarnation to a woman who was already betrothed. In Origen’s reading, then, the marriage of Mary and Joseph is not accidental to the divine plan, but part of it, and so itself becomes theologically significant. It is not Teresa of Ávila, but Origen, who first makes the mystery of St. Joseph and his marriage to Mary an intrinsic part of the mystery of the Incarnation Origen reports:

I found an elegant statement in the letter of a martyr - I mean Ignatius, the second bishop of Antioch after Peter. During a persecution, he fought against wild animals at Rome. He stated, "Mary’s virginity escaped the notice of the ruler of this age." It escaped his notice because of Joseph, and because of their wedding.

Origen continues to ponder the mystery of St. Joseph, pointing out that it is because he is the husband of Mary that the devil does not suspect that the Savior “had taken on a body.” Origen connects the mystery of St. Joseph with Paul’s reference in 1 Cor 2:6-8, where, as Origen reports, Paul comments that:

We speak wisdom among the perfect, but not the wisdom of this age or the wisdom of the rulers of this age. They are being destroyed. We speak God’s wisdom, hidden in a mystery. None of the rulers of this age knows it. If they had known it, they would never have crucified the Lord of glory.

The marriage of Mary and Joseph is thus an intrinsic part of God’s Wisdom, an intrinsic part of the logic of the Incarnation, which is the logic of God’s philanthropia, a logic of foolishness, invisible to the ruling powers because it is, to them, foolishness and not wisdom. Origen invites us to contemplate the marriage of Joseph and Mary as an outcropping, one might say, of this foolishness.