Adam Clarke on Genesis 19:31

Our father is old - And consequently not likely to re-marry; and there is not a man in the earth - none left, according to their opinion in all the land of Canaan, of their own family and kindred; and they might think it unlawful to match with others, such as the inhabitants of Zoar, who they knew had been devoted to destruction as well as those of Sodom and Gomorrah, and were only saved at the earnest request of their father; and probably while they lived among them they found them ripe enough for punishment, and therefore would have thought it both dangerous and criminal to have formed any matrimonial connections with them.