In the Bible ANGELS are CALLED MEN (Gen 18:1-2, 19:1-11), so the fact they were called "men" is really moot and has no bearing on what they were. The strange "nature" of these creatures was unusual, and probably one reason why the Lord was determined to remove every last one of them from the earth, not to mention for the trouble they likely caused.

Nor can it be claimed that they were not offspring of angels because they LOOKED LIKE MEN, because we know that angels can easily assume a human-like form, and that their sex is always masculine (not "SEXLESS"). The writer to the Hebrews mentions that angels can be entertained without man’s knowing it (Hebrews 13:2). Angels must be convincingly like men; the homosexuals of Sodom were very capable of judging sexuality, and they were attracted by the ‘male’ angels who came to destroy the city (Genesis 19:1, especially verse 5).

So we can see that angels are CALLED MEN, and they APPEAR AS MEN in the Bible.

Whatever "nature" they had or did not have, the Bible does define the term ‘the sons of God’ for us.

"Now there was a day when the sons of God came to present themselves before the Lord, Satan also came among them (Job 1:6).

Again there was a day when the sons of God came to present themselves before the Lord, and Satan came among them to present himself before the Lord (Job 2:1).

When the morning stars sang together, and all the sons of God shouted for joy?" (Job 38:7, cf. Psalm 89:6; Daniel 3:25).