Jude 1:7 NASB
7 just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after [g]strange flesh, are exhibited as an [h]example in undergoing the punishment of eternal fire.
But Christ seems to have exonerated these cities ahead of others which had seen his miracles and had heard the gospel preached to them,yet had rejected the gospel.
Matthew 10:15 NASB
15 Truly I say to you, it will be more tolerable for the land of Sodom and Gomorrah in the day of judgment than for that city.
Matthew 11:20-24 NASB
20 Then He began to denounce the cities in which most of His [u]miracles were done, because they did not repent. 21 “Woe to you, Chorazin! Woe to you, Bethsaida! For if the [v]miracles had occurred in Tyre and Sidon which occurred in you, they would have repented long ago in [w]sackcloth and ashes. 22 Nevertheless I say to you, it will be more tolerable for Tyre and Sidon in the day of judgment than for you. 23 And you, Capernaum, will not be exalted to heaven, will you? You will descend to Hades; for if the [x]miracles had occurred in Sodom which occurred in you, it would have remained to this day. 24 Nevertheless I say to you that it will be more tolerable for the land of Sodom in the day of judgment, than for you.”
So how does these cities serve as examples of eternal punishment?