What else would they be? Are you implying that there are celestial beings that reproduce "sons". Man has the capacity to reproduce children, of course. But only one man is called the son of God, that is the original... Adam in Luke 3:38. Adam is an original creation by God.
oikētērion οἰκητήριον = a glorified body
And the angels which kept not their first estate,
but left their own habitation [oikētērion]
he hath reserved in everlasting chains under darkness
unto the judgment of the great day. - Jude 6
For in this we groan, earnestly desiring to be clothed
upon with our house [oikētērion] which is from heaven
2 Corinthians 5:2
Looks like the rebellious angels gave up their supernatural bodies to become manlike,
and then mate with human women, producing half-breed nephilim