Genesis 4:1-4 And after that men began to be multiplied upon the earth, and daughters were born to them, 2 The sons of God seeing the daughters of men, that they were fair, took to themselves wives of all which they chose. 3 …] 4 Now giants were upon the earth in those days. For after the sons of God went in to the daughters of men, and they brought forth children, these are the mighty men of old, men of renown.
"But the angels transgressed this appointment, and were captivated by love of women, and begat children who are those that are called demons…" - St Justin Martyr (AD 100 - AD 165), Second Apology, Ch 5
“These angels, then, who have fallen from heaven, and haunt the air and the earth, and are no longer able to rise to heavenly things, and the souls of the giants, which are the demons who wander about the world, perform actions similar, the one (that is, the demons) to the natures they have received, the other (that is, the angels) to the appetites they have indulged.” - St Athenagoras of Athens (AD 133 – AD 190), A Plea for the Christians, Ch 25
[emphasis mine]