42 It is no excuse for this action of Lot or of his daughters that it represented the perversity which was afterwards in certain cases to be displayed. The purpose of Lot's daughters is one thing, and the purpose of God is another, in allowing this to happen that He might make some truth manifest; for God both pronounces judgment on the actions of the people of those times, and arranges in His providence for the prefigurement of the future. As a part of Scripture, this action is a prophecy; as part of the history of those concerned, it is a crime.
Source: Reply to Faustus the Manichaean (New Advent)