9 This also which follows they like to laugh at, whose wish it is to destroy the authority of the Evangelical books, that they may show as it were that we have without any good reason believed what is said, “When as His mother Mary was espoused to Joseph, before they came together, she was found with Child of the Holy Ghost. Then Joseph her husband being a just man, and not willing to make her a public example, was minded to put her away privily;” for because he knew that she was not with child by him, he thought that she was so to say necessarily an adulteress.
“Being a just man,” as the Scripture says, “and not willing to make her a public example,” (that is, to divulge the matter, for so it is in many copies), “he was minded to put her away privily.” The husband indeed was in trouble, but as being a just man he deals not severely; for so great justice is ascribed to this man, as that he neither wished to keep an adulterous wife, nor could bring himself to punish and expose her. “He was minded to put her away privily,” because he was not only unwilling to punish, but even to betray her; and mark his genuine justice; for he did not wish to spare her, because he had a desire to keep her; for many spare their adulterous wives through a carnal love, choosing to keep them even though adulterous, that they may enjoy them through a carnal desire.
But this just man has no wish to keep her, and so does not love in any carnal sort; and yet he does not wish to punish her; and so in his mercy he spares her. How truly just a man is this! He would neither keep an adulteress, lest he should seem to spare her because of an impure affection, and yet he would not punish or betray her. Deservedly indeed was he chosen for the witness of his wife's virginity: and so he who was in trouble through human infirmity, was assured by Divine authority.
Source: Sermons on Selected Lessons of the New Testament (New Advent)