The Pulpit Commentary on Genesis 19:31

And the firstborn said unto the younger,—showing that she had not escaped the pollution, if she had the destruction, of Sodom. "It was time that Lot had left the cities of the plain. No wealth could compensate for the moral degradation into which his family had sunk" (Inglis)—Our father is old,—an indirect confirmation of the inference (vide Genesis 11:26) that Abram was younger than Haran, since Lot, Haran's son, now an old man—and there is not a man in the earth—not in the entire world (Origen, Irenaeus, Chrysostom, Kalisch), which is scarcely probable, since they knew that Zoar had been spared; but either in the district whither they had fled (Calvin, Willet), being under the impression that, living in so desolate a region, they could have no more intercourse with mankind; or in the land of Canaan (Ainsworth, Bush), meaning that there were no more godly men with whom they might marry; or perhaps they meant that no man would now care to unite himself with them, the remnant of a curse-stricken region (Knobel, Keil)—to come in unto us after the manner of all the earth.