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Giving weight to the theory that "Sons of God" refers to actual angels..in the Old Testament the phrase “sons of God” always refers to angels (Job 1:6; 2:1).

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

And it came to pass, when on a certain day the sons of God came, and stood before the Lord, and Satan came amongst them, and stood in his sight, (Job 2:1)

So, perhaps like a some other form of spiritual being unique to that time?

Or a hybrid being, a perfect intercessor, because they had the spiritual component typical of angels, but also had some kind of capacity to interact with matter.  They would be the ultimate transition between the spiritual realm (God->spiritual angels (higher choirs)->lower choirs->these intermediates->men->animals->matter).  It's always been said that the higher choirs of angels were more purely spiritual somehow than the lower choirs (whatever that means).  They are closer to the pure existence of God, without of course being God themselves.  In fact, one of the Patristic sources said that these were the ones at the first firmament who governed matter ("ruler of matter and its various forms").  Somehow they had the ability to "change their nature" (Pope St. Clement) and assume a corporeal aspect to their nature, perhaps assume these part-spiritual and part-physical bodies (similar perhaps to what we expect of our risen bodies).

Instead of Ancient Aliens, we could call them Ancient Angelions.  So perhaps those guys were right (but wrong at the same time).

One of the interesting aspects of the ancient alien legends is that many of them said that they came down from the constellation Orion ... and others from other constellations.  If you believe these constellations are millions of light years away, then they're aliens.  But if they are, as some of us think, actually up in the firmament, then the one Patristic statement that they came from about the "first firmament" starts to make sense.

Perhaps the ancient stories of these beings that came down from heaven and taught them things, and built the various megalithic structures, founded cultures, etc. ... and then had "heroic" offspring were totally on the mark.  These cultures said they came from the sky.  Greeks had legends about Zeus coming down and impregnating women.  It's easy to write that stuff off as purely made up, but it what if it wasn't totally made up, but a reference to these "sons of God" that came down from the firmament (or down from Olympus or the sky)?