...Early Jewish and Christian commentators agree that what is on view [in Genesis 6:1-4] is a transgression of the boundary between the divine and human realms, angels consorting with human beings.... This being so, the 'unnatural lust' of [Jude] verse 7 (literally 'going after strange flesh') probably refers, not to homosexuality as such, but to the fact that 'the sin of the men of Sodom... reaches its peak when they sought intercourse with angels sent to Lot'.
B. Webb, 'Homosexuality in Scripture'
