A part of an article from Catholic Answers gives this interpretation:
In Luke 1:34, when Mary was told by the angel Gabriel that she was chosen to be the Mother of the Messiah, she asked the question, literally translated from the Greek, "How shall this be since I know not man?" This question makes no sense unless Mary had a vow of virginity.
When we consider that Mary and Joseph were already "espoused," according to verse 27 of this same chapter, we understand Mary and Joseph already have what would be akin to a ratified marriage in the New Covenant. They were married. That would mean Joseph would have had the right to the marriage bed. Normally, after the espousal the husband would go off and prepare a home for his new bride and then come and receive her into his home where the union would be consummated. This is precisely why Joseph intended to "divorce her quietly" (Mt 1:19) when he later discovered she was pregnant.
This background is significant because a newly married woman would not ask the question "How shall this be?" She would know—unless, of course, that woman had taken a vow of virginity. Mary believed the message, but wanted to know how this was going to be accomplished. This indicates she was not planning on the normal course of events for her future with Joseph.
My initial response is, it does not necessarily follow. Why? If Mary had vowed lifelong virginity she would not have married Joseph in the first place. After all, their marriage will only raise the risk of breaking that vow (that is if Mary did make a vow). Unless either spouse is barren (as in the case of Elizabeth in verse 7) no Jewish couple would marry just for companionship or financial stability. Before Gabriel spoke to either couple, neither of them knew that Mary would bear the Messiah. Therefore, Luke 1:34 should be understood in a way that is different from the Roman Catholic interpretation.
The word "know" means sexual relation in this context. Its Greek word, ginosko, is a verb and is in the present tense. Another rendition of the verse is "I am not knowing a man". So presently, Mary is not having any sexual relations with a man. If it were to support the Roman Catholics it should have said, "I will not know a man", rather than "I know not a man." Such has to be clearly written in relation to the prophecy in the Old Testament that a virgin shall be with child. The account of Luke limits itself on this prophecy and does not make any implication about Mary.
Furthermore, although Gabriel's message is in the future tense (1:31, you will be with child), the conception was immediate. Elizabeth is six months pregnant when Gabriel appeared to Mary (Luke 1:36). Mary rushed to Elizabeth in Judea (1:39), stayed with her for 3 months (1:56), and left just before the birth of John the Baptist (1:57). So if the time lapsed between Gabriel's appearance to Mary and the birth of John the Baptist is 3 months, we can conclude that Mary's question (how can this be...?) is an indication that the conception is immediate and not that she made a vow of virginity.
Just like Zechariah who asked how can Elizabeth become pregnant when they are both old (1:18), Mary's question comes from a normal biological means of conception in which a man and a woman should have sexual relations for the woman to bear a child. Therefore, Mary's question in Luke 1:34 is not coming from an idea that she made a vow of virginity.
In Luke 1:34, when Mary was told by the angel Gabriel that she was chosen to be the Mother of the Messiah, she asked the question, literally translated from the Greek, "How shall this be since I know not man?" This question makes no sense unless Mary had a vow of virginity.
When we consider that Mary and Joseph were already "espoused," according to verse 27 of this same chapter, we understand Mary and Joseph already have what would be akin to a ratified marriage in the New Covenant. They were married. That would mean Joseph would have had the right to the marriage bed. Normally, after the espousal the husband would go off and prepare a home for his new bride and then come and receive her into his home where the union would be consummated. This is precisely why Joseph intended to "divorce her quietly" (Mt 1:19) when he later discovered she was pregnant.
This background is significant because a newly married woman would not ask the question "How shall this be?" She would know—unless, of course, that woman had taken a vow of virginity. Mary believed the message, but wanted to know how this was going to be accomplished. This indicates she was not planning on the normal course of events for her future with Joseph.
My initial response is, it does not necessarily follow. Why? If Mary had vowed lifelong virginity she would not have married Joseph in the first place. After all, their marriage will only raise the risk of breaking that vow (that is if Mary did make a vow). Unless either spouse is barren (as in the case of Elizabeth in verse 7) no Jewish couple would marry just for companionship or financial stability. Before Gabriel spoke to either couple, neither of them knew that Mary would bear the Messiah. Therefore, Luke 1:34 should be understood in a way that is different from the Roman Catholic interpretation.
The word "know" means sexual relation in this context. Its Greek word, ginosko, is a verb and is in the present tense. Another rendition of the verse is "I am not knowing a man". So presently, Mary is not having any sexual relations with a man. If it were to support the Roman Catholics it should have said, "I will not know a man", rather than "I know not a man." Such has to be clearly written in relation to the prophecy in the Old Testament that a virgin shall be with child. The account of Luke limits itself on this prophecy and does not make any implication about Mary.
Furthermore, although Gabriel's message is in the future tense (1:31, you will be with child), the conception was immediate. Elizabeth is six months pregnant when Gabriel appeared to Mary (Luke 1:36). Mary rushed to Elizabeth in Judea (1:39), stayed with her for 3 months (1:56), and left just before the birth of John the Baptist (1:57). So if the time lapsed between Gabriel's appearance to Mary and the birth of John the Baptist is 3 months, we can conclude that Mary's question (how can this be...?) is an indication that the conception is immediate and not that she made a vow of virginity.
Just like Zechariah who asked how can Elizabeth become pregnant when they are both old (1:18), Mary's question comes from a normal biological means of conception in which a man and a woman should have sexual relations for the woman to bear a child. Therefore, Mary's question in Luke 1:34 is not coming from an idea that she made a vow of virginity.