In Luke 1 the narrative of Jesus' conception presents Mary in a certain disjunction from Joseph to emphasize her conjunction with God alone. The first character introduced is the angel, to show God's initiative (Lk 1:26). Next the couple Mary and Joseph are presented as a parallel to the preceding narrative of Zechariah and Elizabeth, but in a contrasting manner. The man Zechariah is named first and is the central character who receives the angel's message and is to name the child, while Elizabeth is barren and is mentioned sparsely (Lk 1:5-25). Mary's sociological position is certainly more lowly than that of Zechariah. She is a young girl from Nazareth, a village in Galilee, rather than a priest serving in the temple of the capital city of Jerusalem, and yet she is the one "full of grace," favored by God to conceive the Son of the Most High through the power of the Holy Spirit. In this reversal, the woman is introduced first and she alone is said to be addressed by the angel and commissioned to name the child, while Joseph is barely mentioned (Lk 1:26-38). Mary proclaims this inversion as God's choice of her lowliness over the might of the proud (Lk 1:46-55). The repetition of the word "virgin" in the sentence introducing Joseph (Lk 1:26) and Mary's statement that she has had no sexual relations (Lk 1:34) both emphasize that the conception in her womb is totally God's work in which Joseph has no direct part.
In this context and mode of presentation, however, Joseph is clearly portrayed as "betrothed" to Mary (Lk 1:27; 2:5). The phrase "virgin betrothed to a man" is almost identical to the Greek translation of Deuteronomy 22:23, a passage that seems to be background also for Matthew 1:18-25, which similarly uses the same words for "betrothed" and "virgin." Luke offers no explanation to reconcile this betrothal with divine conception as Matthew does, but in all brevity he presents precisely the same facts: Mary conceives as a virgin while betrothed to Joseph, who is of the house of David. "Betrothed" is again understood to mean that all the formal contractual agreements for the marriage had been finalized, while Mary's statement to the angel indicates that Joseph had not yet made the final step of taking her into his home. Luke 1:34 should be translated "since I have no marital relations with a man," which is a common meaning of the Greek verb "know" when applied to a man and a woman, used also in Matthew 1:25. It should not be translated "since I have no husband," since the betrothed could be called husband and wife even before the act of coming to live together in the same house, as seen in Matthew 1:16,19,20,24.
Luke's intention to present Mary and Joseph's relationship as virginal, at least up through the time of conception, is clear from the verses cited (Lk 1:27,34), and from the angel's explanation about the Holy Spirit coming upon her with the result that her child will be called "holy, the Son of God" (Lk 1:35). The word "firstborn" (Lk 2:6) does not imply that other children were born thereafter, but only that this is the one entitled to the privileges specified by the Mosaic laws, such as the special consecration which will be referred to in the passage on taking Jesus to the temple (Lk 2:23; Ex 13:2).
Mary and Joseph are betrothed at the time, but the physical conception takes place apart from the betrothal, and yet seems to confirm rather than invalidate it. The first function of this betrothal that Luke presents is to root the Messiah in the tree of David, by giving him a lineage traceable through Joseph, who is of the house of David. Luke never explicitly applies the words "husband" or "wife" to Joseph and Mary, but repeats again the word "betrothed" even in the context of their going together to Bethlehem (Lk 2:5). Mary and Joseph are obviously together throughout the birth and the raising of the child and are repeatedly called "parents" together or referred to as "they" or "you" in the plural (Lk 2:6-7,16,22,27, 39,41-46,48-51). For the census Joseph is the sole active character, but he is in conjunction with Mary, who is in conjunction with the Messiah she bears in her womb. The child is associated with Bethlehem, the city of David, through him. Mary's role to give birth and Joseph's role to give Davidic heritage are joined due to their marriage. They are seen as husband and wife during the adoration of the shepherds, during the fulfillment of the legal prescriptions in the temple, during the episode of finding Jesus in the temple at the age of twelve, and during the years at Nazareth when Jesus was obedient to them. They provide the unity and affection for the Son of the Most High to be raised in an ordinary human household. Without employing the terms "husband" and "wife" used by Matthew, Luke presents a more detailed portrayal of their acting as such during Jesus' infancy and into his adolescence.
Taken from http://www.osjoseph.org/stjoseph/scripture/part2sec3.php