Louw Nida identifies the whole phrase "ἀπέρχομαι ὀπίσω σαρκὸς ἑτέρας" (‘to go after strange flesh') as being idiomatic in nature noting that "Though in some societies homosexuality is extremely rare, there are always ways of talking about it, though frequently the expressions may seem to be quite vulgar." [Louw, J. P., & Nida, E. A. (1996). Greek-English lexicon of the New Testament: based on semantic domains (electronic ed. of the 2nd edition., Vol. 1, p. 771 (88.279)]. However the lexicon does not provide any support for this assertion that the phrase is idiomatic of homosexual behavior.
I am not personally aware of any other places in Greek or Hebrew literature where this exact idiom is employed - it seems that the more common idiom was natural/ unnatural however I do believe there are some very good reasons to understand this phrase as referring to homosexual activity of some description rather then a desire to have intercourse with Angels.
The word 'strange' as used in some versions (KJV, NKJV, NASB etc) may, potentially create a different meaning in the mind of modern English readers to that intended by the Greek adjective ἑτέρας. The Greek term basically means 'other' or 'different' however Robertson notes that:
The sense of “different” grows naturally out of the notion of duality.
The two things happen just to be different…. The word itself does not
mean “different,” but merely “one other,” a second of two. It does not
necessarily involve “the secondary idea of difference of kind”
(Thayer). That is only true where the context demands it1/
It should be noted that in Acts 2:4 it is used to refer to other tongues (or human languages) and in Acts 20:15 it refers to the next day - in these usages the idea of a difference of kind is alien to the use of the word. The term itself then does not have to mean something entirely different. In context it seems to mean 'other' rather then that which was'normal' or different to that which is normal.
This interpretation is entirely in keeping with first century Jewish norms. Josephus and Philo not only condemn relations that are “contrary to nature,” they explicitly understand Genesis 19 as referring to homosexual acts2 and the teaching of scripture (Gen 1:26-28; 2:21-24; Lev 18:1-30; Matt 19:3-6; Mark 10:2-9)
Indeed it would be very strange for Jewish writer to refer to Angels using the Greek term σαρκὸς. Of all the potential ways to refer to angels a term used to refer to the physical, earthly and human would be a very strange choice. However using the sin of Sodom and Gomorrah as a striking example of the immortality of homosexual behavior would certainly be relevant in the first century Greco-Roman context to warn against such behavior as opposed to the non-existent temptation to have sex with angels.
It must also be noted that the men of Sodom did not know that the visitors Lot received were Angels. In Gen 19:5 we read them demanding that Lot bring out the 'men' that had come to visit. It is not even clear that Lot had identified them as Angels, they had the appearance of men (Gen 18:2,16,22; 19:1,5,8,10,12,16), whose feet could be washed (Gen 19:2) and who could eat (Gen 19:3).
In similar vein we must also note the beginning of Jude 7 where we are told, "...as Sodom and Gomorrah, and the cities around them...." the sin that Jude is referring to is more then a single occurence that happened that one night (when only the male inhabitants of Sodom were involved - gen 19:4) - it seems only logical to understand the sin been referred to as a lifestyle of sensuality and sexual immorality, at least one aspect of which was exemplified in men pursuing the flesh of other men instead of the flesh of women as many conservative and evangelical scholars agree:
If we look again at the literal translation of verse 7a, we are able
to read the text as follows: “How Sodom and Gomorrah (and the cities
surrounding them in a similar way to these) were indulging in sexual
immorality and went after other flesh.” In the context of the verse,
the position of the pronoun these indicates that Jude refers to the
men of Sodom.35 But what is the meaning of the term other flesh? The
Greek reveals that in the case of duality (for example, male and
female) the word other can mean “a second of two” and in the context
denote a difference of kind.36 Therefore, when the men of Sodom were
interested in sexual relations with men, they perverted the created
order of natural intercourse. That is, the men of Sodom did not desire
females (see Gen. 19:8–9); instead, these men demanded homosexual
relations with the men who visited Lot. The activity of the Sodomites
is perversion. This is precisely how translators of the New
International Version render the phrase went after other
Further more if one separates Jude 1:6 from the events of Gen 6:1-8 which is entirely possible as Jude mentions nothing of angels coming to earth and procreating with human beings (and in this writers opinion such a conclusion is probable for that interpretation of Gen 6:1-8 is problematic) then the whole concept of linking sexual immorality with Angelic beings is lost from the passage leaving us to return once again to simplest understanding of what the term must mean - the clearly documented behaviors that were practiced in Sodom and the surrounding cities of the Jordan plain4.
1/ A.T. Robertson, A Grammar of the Greek New Testament (1934, p. 748)
2 Josephus writes "And when God had replied that there was no good man among the Sodomites; for if there were but ten such men among them, he would not punish any of them for their sins, Abraham held his peace. And the angels came to the city of the Sodomites, and Lot entreated them to accept of a lodging with him; for he was a very generous and hospitable man, and one that had learned to imitate the goodness of Abraham. Now when the Sodomites saw the young men to be of beautiful countenances, and this to an extraordinary degree, and that they took up their lodgings with Lot, they resolved themselves to enjoy these beautiful boys by force and violence; (201) and when Lot exhorted them to sobriety, and not to offer anything immodest to the strangers, but to have regard to their lodging in his house; and promised, that if their inclinations could not be governed, he would expose his daughters to their lust, instead of these strangers—neither thus were they made ashamed." [Ant. 1.200-201] and Philo writes "and so, by degrees, the men became accustomed to be treated like women, and in this way engendered among themselves the disease of females, and intolerable evil; for they not only, as to effeminacy and delicacy, became like women in their persons, but they made also their souls most ignoble, corrupting in this way the whole race of man, as far as depended on them" [T. ab. 136]
3 Kistemaker, S. J., & Hendriksen, W. (1953–2001). Exposition of the Epistles of Peter and the Epistle of Jude (Vol. 16, pp. 381–382). Grand Rapids: Baker Book House.
4 The sin of Sodom involved more than homosexuality (Eze 16:49-50) however in Jude 1:7 the writer is referring to sexual immorality.