Genesis 19:31

And the firstborn said unto the younger
That is, the firstborn of those two, or the elder of them; for, if Lot had other daughters that were married in Sodom, it is probable they were elder than either of these: Aben Ezra intimates, that Lot had another wife, who died first, and these were by his second; the following motion is made by the eldest of them to the youngest, as being bolder, having more authority, and a greater influence to persuade:

our father [is] old;
if he was fifty years of age when he was taken captive by the kings, as says the Jewish chronologer ^F17 he must now be sixty five, since the destruction of Sodom, according to Bishop Usher ^F18, was fifteen years after that:

and [there] is not a man in the earth to come in unto us after the
manner of all the earth;
to marry them, cohabit with them, and procreate children of them, which was the common way of the propagation of mankind in the earth; they thought the whole world was destroyed by fire, as it had been by a flood; they understood it would be no more consumed by water, but they had been told it would be by fire, and they imagined the time was now come, and this was the case; that not only Sodom and Gomorrah were destroyed by fire, and that by this time the fire had reached to Zoar, and had consumed that, but that the whole earth was destroyed, and not a man left but their father, and therefore thought it could be excusable in them, and lawful for them to take the following method to repopulate the world; or else they supposed there were none in the land, the land of Canaan, not of any of their kindred and relations, for they might be ignorant of Abraham and his family, or however of any good man that they knew of, that they could be joined to in marriage; for as for the inhabitants of Zoar, they had just left, they were as wicked as any, and therefore could not think of living with them in such a near relation: but all this is not a sufficient excuse for contriving and executing what is after related; for they should have inquired of their father, who could have informed them better.