1 Enoch 6:2
1 And it came to pass when the population of humans had increased during those times, beautiful and attractive daughters were born to them. 2 And the angels, the children of heaven, saw them and desired them, and said to each other: 'Come, let us choose wives from among the humans and father children.' 3 And Semjâzâ, their leader, said to them: 'I fear that you will not actually agree to do this, and I alone will have to pay the penalty of a great sin.' 4 And they all replied to him and said: 'Let us all take an oath, and all bind ourselves with a solemn promise not to abandon this plan but to carry out this act.' 5 Then they all took an oath together and bound themselves with a solemn promise to do so.
Josephus Antiquities of the Jews 1.3
1 Now this posterity of Seth continued to esteem God as the Lord of the universe, and to have an entire regard to virtue, for seven generations: but in process of time they were perverted, and forsook the practices of their fore-fathers; and did neither pay those honours to God which were appointed them, nor had they any concern to do justice towards men. But for what degree of zeal they had formerly shewn for virtue, they now shewed by their actions a double degree of wickedness. Whereby they made God to be their enemy. For many Angels of God accompanied with women, and begat sons that proved unjust, and despisers of all that was good; on account of the confidence they had in their own strength. For the tradition is, that these men did what resembled the acts of those whom the Grecians call Giants. But Noah was very uneasy at what they did: and being displeased at their conduct, persuaded them to change their dispositions, and their actions for the better. But seeing they did not yield to him, but were slaves to their wicked pleasures, he was afraid they would kill him, together with his wife and children, and those they had married. So he departed out of that land.
Notes and References
"... In a few instances, interpreters of Israel’s scriptures view ‘sons of God’ as a reference to angelic beings (Stuckenbruck 2014). Beginning with the infamously arresting passage in Genesis 6:1–4, the ‘sons of God’ mate with the daughters of humanity to produce a renowned race of giants. For early interpreters, including 1 Enoch (6:2 in Codex Panopolitanus), Philo (Questions and Answers in Genesis 1.92; Philo 1993), and Josephus (Antiquities 1.3.1.73; Josephus 1926), these ‘sons of God’ were angels. Outside of antediluvian history, some Greek versions of Deuteronomy 32:43 invite the angelic sons of God to praise the Lord. Several instances of angels as ‘sons’ also appear in the Psalms, including Psalm 28:1 and Psalm 88:6, where again they are depicted as praising God. Psalm 81 depicts a heavenly court scene where the sons of God are punished with death (Psalm 88:6), which early Christian interpreters saw as a description of the rebellious angels’ punishment. This association between sons and angels seems clear in the early Hebrew portions of Job, where the ‘sons of God’ present themselves to God as a sort of heavenly council (Job 1:6; 2:1). Later these sons are mentioned as present with God at the beginning of creation (Job 38:7). Although a minor theme, angelic beings are therefore included within the category of ‘sonship’ ..."
Peeler, Amy Sonship in the Bible (pp. 1-20) St Andrews Encyclopaedia of Theology, 2022