Genesis 19:31

And the elder said to the less, Our father is old, and there is no man in the land to come to us after the way of the whole earth:

AND THE FIRST-BORN SAID. Rabbi Abraham ibn Ezra said, “It is possible that Lot had another wife It must therefore be, concluded Ibn Ezra, that the ones mentioned above were from another wife who had died. Ramban, however, differs with this suggestion, as explained below in the text. who died before.”
But there is no need for this; the first-born is just in contrast to the younger. In fraternal relations the one who is older is called “first-born,” and all those younger than he are called “the younger ones.”
Thus the first fruits of the year are called bikurim; has as its root the word b’chor (the first born). Here also the first fruits are relative to this year’s crop. likewise, the first-born of the poor, meaning the most destitute, the poor of the poor. Likewise, With the loss of his first-born shall he lay the foundation thereof, and with the loss of his younger shall he set up the gates of it. Onkelos also here translated, rabtha. (the first-born).

ואיש אין בארץ, they thought that the whole universe had been turned upside down and that G’d had brought on a deluge of fire.

ותאמר...אבינו זקן, there is no chance that he will marry again and have children from such a woman. If we will die without having children there will not be a memory of our father at all. If you were to say that we should take husbands and have children, where would we find such husbands?

איש אין בארץ לבא עלינו. Most interpreters assume that these daughters thought that that just as there was no survivor left from the cities of Sodom and Gomorrah, so there were none in the rest of the world. It is very difficult to accept such an interpretation, seeing that they had just left Tzoar which had not been destroyed, so that they had every reason to believe that other regions of the earth had also not been affected. Not only that, but their father had told them that the reason why Sodom and Gomorrah had been destroyed was due to their inhabitants’ wickedness. I therefore prefer an explanation which I have heard in the name of Rabbi Yoseph Karo (the elder) that the elder sister told her younger sister that none of the remaining men of the world would want to marry them, seeing they had lived in the wicked city of Sodom. People would not want to associate with anyone who had only narrowly escaped the fate of the Sodomites.

ואיש אין בארץ לבא עלינו “and there is no other man left on earth to impregnate us with his sperm.” According to Ibn Ezra, they did not think that there were no more males alive on earth, but they thought no other man would be a suitable marriage partner for them for spiritual reasons. Since they had been miraculously saved by G–d, they considered themselves as of superior status.