John Calvin Commentary Jude 1:7

"Even as Sodom and Gomorrah, and the cities about them, having in like manner with these given themselves over to fornication and gone after strange flesh, are set forth as an example, suffering the punishment of eternal fire." — Jude 1:7 (ASV)

Even as Sodom and Gomorrha. This example is more general, for he testifies that God, excepting no one, punishes without distinction all the ungodly. And Jude also mentions later that the fire through which the five cities perished was a type of the eternal fire. So, God at that time exhibited a remarkable example, to keep people in fear until the end of the world. This is why it is so often mentioned in Scripture; indeed, whenever the prophets wished to designate some memorable and dreadful judgment of God, they depicted it using the image of sulfurous fire and alluded to the destruction of Sodom and Gomorrha. Therefore, it is not without reason that Jude strikes all ages with terror by exhibiting the same view.

When he says, the cities about them in like manner, giving themselves over to fornication, I do not apply these words to the Israelites and the angels, but to Sodom and Gomorrha. It is no objection that the pronoun τούτοις is masculine, for Jude refers to the inhabitants and not to the places. To go after strange flesh, is the same as to be given up to monstrous lusts; for we know that the Sodomites, not content with the common way of committing fornication, polluted themselves in the most filthy and detestable way. We should observe that he condemns them to eternal fire, for from this we learn that the dreadful spectacle which Moses describes was only an image of a much heavier punishment.