‘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.'
The third incident cited is the case of the sin of Sodom and Gomorrah, cities which had sunk into deep sin, corruption, lasciviousness and illicit sex. This had caused God to send angels to inspect them, and resulted in an attempt by the men of Sodom to rape the angels (to ‘go after strange flesh').
Here was an example of lasciviousness and illicit sex, and a further attempt to break down the barrier between the heavenly and the earthly by the rape of angels. Alternately ‘strange/other flesh' may signify ‘that which was not usual' and may therefore indicate ‘man with man doing what is unseemly' (Romans 1:27).
And what was the consequence? The result was that they were made an example of, and suffered the punishment of eternal fire. The idea behind the phrase ‘eternal fire' might be that it was fire from heaven (the eternal regions), or it may refer to its everlasting consequences, or it may refer to both. Jude also no doubt expects his readers to consider the eternal fire yet to come (Mark 9:43-48; Matthew 25:41; Revelation 19:20; Revelation 20:10; Revelation 20:14).
So these three incidents make clear the consequence of unbelief:
1) In the light of the demands of the one God and the offer of the One Saviour.
2) In consequence of men trying to involve themselves with spirit beings.
3) And in consequence of men living lives of lasciviousness and illicit sex.
All such sins bring men into destruction and judgment.
We should note in the present day that this has in mind belief in numerous intermediaries in relation to God, ‘modern' attitudes to illicit sex (which are simply a repetition of the ancient sins) and modern attempts to become involved with the occult in whatever way it is done, whether by mediums, ouija boards, tarot cards, crystal balls, or whatever. All are equally condemned.