43 At the same time there is in this transaction no reason for the torrent of abuse which Faustus' blind hostility discharges on it. By the eternal law which requires the preservation of the order of nature and condemns its violation, the judgment in this case is not what it would have been if Lot had been prompted by a criminal passion to commit incest with his daughters, or if they had been inflamed with unnatural desires. In justice, we must ask not only what was done, but with what motive, in order to obtain a fair view of the action as the effect of that motive. The resolution of Lot's daughters to lie with their father was the effect of the natural desire for offspring in order to preserve the race; for they supposed that there were no other men to be found, thinking that the whole world had been consumed in that conflagration, which, for all they knew, had left no one alive but themselves. It would have been better for them never to have been mothers, than to have become mothers by their own father. But still, the fulfillment of a desire like this is very different from the accursed gratification of lust.
Source: Reply to Faustus the Manichaean (New Advent)