45 But we are defending the sacred Scriptures, not man's sins. Nor are we concerned to justify this action, as if our God had either commanded it or approved of it; or as if, when men are called just in Scripture, it meant that they could not sin if they chose. And as, in the books which those critics find fault with, God nowhere expresses approval of this action, what thoughtless folly it is to bring a charge from this narrative against these writings, when in other places such actions are condemned by express prohibitions!
In the story of Lot's daughters the action is related, not commended. And it is proper that the judgment of God should be declared in some cases, and concealed in others, that by its manifestation our ignorance may be enlightened, and that by its concealment our minds may be improved by the exercise of recalling what we already know, or our indolence stimulated to seek for an explanation. Here, then, God, who can bring good out of evil, made nations arise from this origin, as He saw good, but did not bring upon His own Scriptures the guilt of man's sin. It is God's writing, but not His doing; He does not propose these things for our imitation, but holds them up for our warning.
Source: Reply to Faustus the Manichaean (New Advent)