Jude 4 describes the sin of the opponents as ἀσέλγεια, a word which has the sense of abandoning the restraints of socially accepted behavior, almost always sexual sin. (Only 2x in the LXX: Wisdom of Solomon 14:26, “sexual perversion”, 3 Maccabees 2:26 uses the word to describe the sexual excess of the Greek king of Egypt; cf., T.Levi 23:1, “lewdness.”)
Some of Jude’s biblical illustrations for these opponents are sexually oriented as well. The fallen angels (Gen 6:1-4, 1 Enoch). The sexual nature of the sins of the angels in Gen 6 is more clear in the 1 Enoch version, perhaps explaining Jude’s use of the more legendary form of the story. Sodom and Gomorrah (Gen 14) were legendary for their sexual sins, it is possible that Jude has general sexual excess in mind rather than homosexuality.
Taken along with Jude’s reference to the opponents being “blemishes” on the church’s love feasts, it is likely that these teachers were using church meals as an opportunity for sexual debauchery. While this sounds completely alien to the later church, in a Greco-Roman context this makes more sense. Greco-Roman banquets were known for not only over indulgence in good food and wine. A good meal was often followed by sexual encounters with prostitutes.
Paul dealt with this very problem in Corinth where was a problem with gluttony, drunkenness and going to prostitutes at private banquets (1:Cor 6:12-20). The issue here is attendance at banquets given by the rich elite of the city. There is plenty of evidence concerning the types of things that went on in a Roman banquet of the first century from contemporary writers.
If this is the background for the opponents in Jude, then once again we have evidence for an earlier date to the book, and perhaps another indication that the problems were caused by people, perhaps Jews, failing to challenge their pagan world with their new faith. I suspect that this is one of the more applicable elements of the book of Jude. These “Christians” are using their religion to promote behaviors which would even shock the Romans.