Readers Knew Sodom's Angelic Visitors

Jude does not explain himself. He writes "other flesh" and moves on, which tells you something about the people he was writing to: they did not need it explained.
His readers came out of a Jewish world that read Genesis 6 and Genesis 19 together as a matched pair - heavenly beings crossing into the human realm, and humans reaching back into the heavenly one. Within that framework the phrase is not cryptic at all. It is precise. It is only obscure to us because we inherited a reading tradition that dropped the background.
Scripture links:
- Jude 7 - "pursued unnatural desire" - literally, went after other flesh.
- Genesis 19:1 - The visitors are angels, stated plainly.
When a New Testament writer does not explain a phrase, that is evidence about his audience. Jude assumed you already knew.