Sermon of St Jerome
Why was the Lord conceived of a virgin espoused rather than of one who was not? First, that Mary's genealogy might be reckoned from that of Joseph. Secondly, lest she be stoned by the Jews as an adulteress. Thirdly, that she might have a guardian on their flight into Egypt. To these, the Martyr Ignatius hath added a fourth reason; namely, that the birth might take place unknown to the devil, who would thus suppose that Mary had conceived by Joseph.
Before they came together, she was found with child, of the Holy Ghost. That is, she was found by Joseph, not by anyone else, for already he had almost an husband's privilege to know all that concerned her. But from the words, Before they came together, it doth not follow that they ever did come together. The Scripture is concerned only to shew that up to this time they had not so done.
Then Joseph her husband, being a just man, and not willing to make her a public example, was minded to put her away privily. If any man be joined to an harlot, he becometh one body with her; and according to the law, they that be privy to a crime are held to be guilty. How then can it be that Joseph is described as a just man, at the very time he was compounding the criminality of his espoused? These words be none other than a testimony to the virginity of Mary; for Joseph knew her to be chaste; wherefore he marvelled at all that had come to pass, and hid in silence that of which he knew not the mystery.