the term, in the Book of Jude, for what they did is the same SIN as what happened with Sodom.
Strange Flesh:
6 And the angels—who did not keep their own position of authority but deserted their proper place—He has kept in everlasting shackles under gloomy darkness until the judgment of the great Day.
7 In the same way as these angels, Sodom and Gomorrah and the cities around them—having given themselves over to sexual immorality and gone after a different sort of flesh—are displayed as an example, suffering the punishment of eternal fire.
now:
Notice Verse 6:
6 And the angels—who did not keep their own position of authority [THESE ARE COUNCIL MEMBER ANGELS WHO REBELLED WITH LUCIFER]
Notice Verse 7 the Angels SIN Committed:
7 In the same way as these angels, Sodom and Gomorrah and the cities around them—having given themselves over to sexual immorality and gone after a different sort of flesh.
Notice specifically what these Council Member Angels did:
having given themselves over to sexual immorality and gone after a different sort of flesh