The Gospel reading from today's Mass is Luke 1:18-25. In the commentary on verse 25, I came across this rich theological and mariological (is that word?) tidbit the editors pointed out:
St. Jerome gives the following reasons why it was fitting that the Mother of God, as well as being a virgin, should also be married: first, so that Mary's child would be clearly a descendant of King David (through the genealogy of St. Joseph); second, to ensure that on having a son her honour would not be questioned nor any legal penalty be imposed on her; third, so that during the flight into Egypt she would have the help and protection of St. Joseph.
This, I thought, was particularly interesting:
He even points to a fourth possible reason, expressly taken from St. Ignatius Martyr, and to which he seems to give less importance—that the birth of Jesus would go unnoticed by the devil, who would not know about the virginal conception of our Lord (cf. Comm. on St Matthew, 1, 1).