Rashi on Genesis 19:31/רש"י על בראשית י״ט:ל״א

1אבינו זקן OUR FATHER IS OLD — And if not now, when? He may die or may cease to beget children.

2ואיש אין בארץ AND THERE IS NOT A MAN IN THE EARTH — They thought that the whole world had been destroyed as in the time of the generation of the Flood. (Genesis Rabbah 51:8).