According to the book of Enoch. When the war in heaven started, the angels who rebelled were cast to the Earth.
While on Earth, the Angels ( now referred to as Watchers ) decided to lay with human women!
This is a quote from chapter 15 Verses 2 through 10.
2. " Wherefore have you forsaken the lofty holy heaven, which endures for ever, and have lain with women; have defiled yourselves with the daughters of men; have taken to yourselves wives; have acted like the sons of the earth, and have begotten an impious offspring?
3. You being spiritual, holy, and possessing a life which is eternal, have polluted yourselves with women; have begotten in carnal blood; have lusted in the blood of men; and have done as those who are flesh and blood do.
4. These however die and perish.
5. Therefore have I given to them wives, that they might cohabit with them; that sons might be born of them; and that this might be transacted upon earth.
6. But you from the beginning were made spiritual, possessing a life which is eternal, and not subject to death for ever.
7. Therefore I made not wives for you, because, being spiritual, your dwelling is in heaven.
8. Now the giants, who have been born of spirit and of flesh, shall be called upon earth evil spirits, and on earth shall be their habitation. Evil spirits shall proceed from their flesh, because they were created from above; from the holy Watchers was their beginning and primary foundation. Evil spirits shall they be upon earth, and the spirits of the wicked shall they be called. The habitation of the spirits of heaven shall be heaven; but upon earth shall be the habitation of terrestrial spirits, who are born on earth.
9. The spirits of the giants shall be like clouds, which shall oppress, corrupt, fall, contend,and bruise upon the earth.
10. They shall cause lamentation. No food shall they eat; and they shall be thirsty; they shall be concealed, and shall not rise up against the sons of men, and against women; for they come forth during the days of slaughter and destruction."
God said it, it is so! Amen!