How are the words of Mary in Luke 1:34 incontrovertible proof of her perpetual virginity?
Luke 1:34 NIV How will this be,” Mary asked the angel, “since I am a virgin?”
"Catholics make a big deal out of this verse declaring it to be proof that Mary was informing Gabriel that she was a woman who had vowed to remain a virgin forever."
Well, before going on into the main response to this question, please allow me to make note of the entire dialogue between the Angel Gabriel and Mary, the Mother of Jesus in the Gospel of St. Luke:
The Birth of Jesus Foretold
26 In the sixth month of Elizabeth’s pregnancy, God sent the angel Gabriel to Nazareth, a town in Galilee, 27 to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin’s name was Mary. 28 The angel went to her and said, “Greetings, you who are highly favored! The Lord is with you.”
29 Mary was greatly troubled at his words and wondered what kind of greeting this might be. 30 But the angel said to her, “Do not be afraid, Mary; you have found favor with God. 31 You will conceive and give birth to a son, and you are to call him Jesus. 32 He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, 33 and he will reign over Jacob’s descendants forever; his kingdom will never end.”
34 “How will this be,” Mary asked the angel, “since I am a virgin?”
35 The angel answered, “The Holy Spirit will come on you, and the power of the Most High will overshadow you. So the holy one to be born will be called[a] the Son of God. 36 Even Elizabeth your relative is going to have a child in her old age, and she who was said to be unable to conceive is in her sixth month. 37 For no word from God will ever fail.”
38 “I am the Lord’s servant,” Mary answered. “May your word to me be fulfilled.” Then the angel left her. - (Luke 1:26-38)
The Church seldom, if ever, takes one unique source to base a single doctrine on. The perpetual virginity is likewise in this category. It is quite obvious how someone who is a virgin but is betrothed to a man (as Mary was) could conceive a child. There would be certainly no need to ask the Angel Gabriel about this. On the other hand, it is not obvious how someone who, though betrothed, intended to remain a virgin could conceive a child. That situation would warrant asking Gabriel to explain. Mary's asking Gabriel this question makes sense only if she intended to remain a virgin.
The relevant verse is Luke 1:34: “How shall this be, seeing I do not know man.” These words of Mary to the Angel Gabriel at the Annunciation show that Mary did not intend to have conjugal relations with a man; otherwise, Mary surely would have known that conjugal relations with Joseph, her husband, could cause a pregnancy. Catholic theologian Stefano Manelli explains Mary’s strange response to the angel this way:
“Confronted by this [the angel Gabriel’s] wondrous announcement, however, the virgin finds herself embarrassed; not because of the sublime greatness of the majesty announced to her, but rather for the way in which such a maternity might be realized. The embarrassment would seem inexplicable because, on any reasonable grounds, she is precisely a woman in ideal conditions to conceive a son. She is the young spouse of Joseph – What young spouse would not be inclined to desire a beautiful son? It is obvious, therefore, and must be acknowledged that Mary’s difficulty stems from a precise commitment — vow or promise — “not to know man,” that is, to be and remain a virgin. St. Augustine rightly says, that ‘Mary certainly would not have spoken those words If she had not vowed her virginity to God” In fact, only by admitting Mary’s virginal consecration to God, can it be understood why she found herself facing an unsolvable dilemma: How to reconcile her virginal offering to God with the request of maternity on the part of God? How could she become a mother without betraying a promise of virginal consecration to God.” (Stefano Manelli, All Generations Shall Call Me Blessed, pages 137-140) - THE BIBLICAL BASIS FOR MARY’S PERPETUAL VIRGINITY
Seeing that Mary was overshadowed by the Holy Spirit, she became the spouse of the Holy Spirit and was the formal cause of her virginal conception of Jesus (Luke 1:35).
Mary, Spouse of the Holy Spirit
Mary is properly to be considered the spouse of the Holy Spirit since the Holy Spirit “overshadowed” her and was the formal cause of her virginal conception of Jesus (Luke 1:35). This is why her offspring, Jesus, “will be called the Son of God” (Luke 1:35), and this is also why Mary is properly called the Mother of God according to the decree of The Council of Ephesus in 431. Mary’s unique and “supernatural maternity” through the power of the Holy Spirit necessarily precludes her from intimate union with a man. Mary is a virgin because of her “undivided gift of herself” to God (Catechism of the Catholic Church, 506). - THE BIBLICAL BASIS FOR MARY’S PERPETUAL VIRGINITY
Now Mary did not tell Joseph that she was pregnant with the Son of God. In her humility and abandonment she left that to her God:
Joseph Accepts Jesus as His Son
18 This is how the birth of Jesus the Messiah came about: His mother Mary was pledged to be married to Joseph, but before they came together, she was found to be pregnant through the Holy Spirit. 19 Because Joseph her husband was faithful to the law, and yet did not want to expose her to public disgrace, he had in mind to divorce her quietly.
20 But after he had considered this, an angel of the Lord appeared to him in a dream and said, “Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit. 21 She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins.”
22 All this took place to fulfill what the Lord had said through the prophet: 23 “The virgin will conceive and give birth to a son, and they will call him Immanuel” (which means “God with us”).
24 When Joseph woke up, he did what the angel of the Lord had commanded him and took Mary home as his wife. 25 But he did not consummate their marriage until she gave birth to a son. And he gave him the name Jesus. - Matthew 1:18-25
Both Joseph and Mary were privileged persons in the mystery of Our Lord's birth and Our Lady's perpetual virginity. Not a single word from St. Joseph is ever recorded in the Scriptures. Like Mary he too must have pondered all these things in his heart.
The relevant verse is sometimes translated, “he [Joseph] had no relations with her [Mary] until she bore a son, whom he named Jesus.” The New American Bible translates the verse as follows: “He had no relations with her at anytime before she bore a son, whom he named Jesus,” This verse demonstrates that Joseph did not have sexual relations with Mary before Jesus’ birth, thus establishing the doctrine of Jesus’ virginal birth, The verse does not mean that Joseph had sexual relations with Mary after Jesus was born. - THE BIBLICAL BASIS FOR MARY’S PERPETUAL VIRGINITY
The Protoevangelium of James, written around A.D. 120, had as one of its principal aims to prove the perpetual virginity of Mary. Origin (died 254) strongly defended Mary’s perpetual virginity but Tertullian (died 230), who was excommunicated, denied it. Other early Church fathers affirming Mary’s perpetual virginity include Athanasius, Epiphanius, Jerome, Ambrose, Augustine and Cyril of Alexandria.