This lection is, of course, one of the prime passages used and preached on during the Christmas season. The challenge is to say something fresh but yet familiar and reassuring about it.
An important exegetical perspective that needs to be kept in mind is the Matthean text tells the story more from the angle of Joseph's perspective, while the Lukan birth narrative tells the tale from the perspective of how things affected and were seen by Mary. What the two narratives have in common is interesting: 1) a birth in Bethlehem, even though the family is from Nazareth and Jesus would be called Jesus of Nazareth; 2) a virginal conception; 3) a pregnancy during the engagement period caused through the agency of the Holy Spirit; and 4) Joseph resolves to accept Jesus into his life and family, as is shown by subsequent events.
Though it has become fashionable in some scholarly circles to suggest the story of the miraculous conception of Jesus has analogies with the stories about the births of Emperors or Kings, in fact this is not really accurate. A story about a god coming down and raping a human woman is of a very different ilk than the story of a miraculous virginal conception through the power of the Holy Spirit, not through any sort of intercourse.
Furthermore, the story in Isaiah 6 about a virgin conceiving, while compatible with our story in Matthew 1, does not in fact specify a virginal conception. It simply says a nubile woman of marriageable age, who was indeed a virgin, would conceive and give birth to a child. Unlike Matthew 1, that text does not specify the means by which the virgin is impregnated, and all indications are that early Jewish were not looking for, nor did they think, Isaiah 6 predicted a miraculous conception.
This explains the shocked reaction of both Joseph in Matthew 1 and Mary in the Lukan account. The assumption a Torah-true Jew like Joseph must have made is Mary got pregnant in the usual manner, hence his decision to divorce her quietly. It took further divine intervention in the form of a dream to head off that disaster, and the disgracing and shaming (not to mention the potential stoning) of Mary. In short, the potential scandal in this story, and the lack of a clear prediction of a virginal conception in Isaiah 6 or parallel in other birth narratives, means this story arose from an historical incident in the life of Mary and Joseph, and then was explained with the aide of the text of Isaiah 6. The First Evangelist uses Isaiah to provide proof that this surprising and unprecedented event was, in fact, a fulfillment of Scripture and all along a part of God's plan for human redemption.
Some background information about early Jewish marriages helps the exposition of this text. In the first place, engagement in this culture was a formal contractual matter, usually decided on by the two fathers in question (i.e. it was an arranged marriage), and was, in fact, the first stage of the marriage itself, to be complete some months hence by the formal wedding ceremony. The reason Matthew says that Joseph had resolved to "divorce" a woman he was only engaged to, is because engagement then was a legally binding contract, unlike engagement in the West today.
Secondly, we need to understand in that patriarchal culture, the birth of the first born son was all important and crucial to the family line and property transfer. The fact Joseph is prepared to give up the right to sire his own first born son and accept and even name Jesus (Yeshua/Joshua means "Yahweh saves") says a lot about the character of Joseph. It leads to the oddest genealogy ever in Matthew 1:1-17 in which Jesus is shoehorned into Joseph's genealogy by putting Mary into that genealogy despite the fact that it is a patrilineal genealogy (x begat y...).
This is a narrative of surprising and unexpected events and suggests a God of unexpected actions. Finally, Matthew 1:25 is a crucial conclusion to our passage and suggests Mary and Joseph did not have marital relations until after the birth and naming of Jesus. The stories thereafter (see e.g. Mark 3:21-35 and Mark 6 and the parallels in Matthew) suggest Mary and Joseph, being good early Jews, went on to have numerous children, both boys and girls the natural way who are rightly called Jesus' brothers and sisters. In short, Matthew's Gospel affirms the virginal conception of Mary, but not her perpetual virginity, or for that matter her own immaculate conception by her mother. Those ideas are found only in much later Catholic traditions.