I like how this adds to what i found. It seems to fit, based on the fact that they needed to resort to such an offer.... which would spare the angels from the same punishment as those mentioned in Jude 6.
Jude 6-7: And the angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day; as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the vengeance of eternal fire.
It likens the actions of the chained angels to those of Sodom and Gomorrah, so clearly those angels mentioned were guilty of committing the same types of evils as S&G, sexual immorality and going after a strange flesh.... which is why these angels are in chains. So it is of a sexual nature.