That's only part of the context. I showed you the full context in this post:
“The angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day, just as Sodom and Gomorrha and the cities about them, in like manner giving themselves over to fornication, and going after strange flesh, are set forth for an example, will suffer the vengeance of eternal fire.”
"Fornication" = "ekporneo" ἐκπορνεύω (Greek for "indulge in illicit sexual relations")
It was sinful