The following is written in the Old Testament of the Bible, the book of Genesis:

"When men began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of men were beautiful, and they married any of them they chose. Then the LORD said, 'My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years.' The Nephilim were on the earth in those days-and also afterward-when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown."

(Genesis 6:1-4 NIV)

There are Bible scholars who believe that the "sons of God" spoken of in (Genesis 6:1, 4) refer to fallen angels (the term "sons of God" is often used in the Scripture to refer to angels). Their belief is that the Nephilim (definition: the fallen ones) spoken of in (Genesis 6:4) were a race of giant demigods produced by the unnatural cohabitation between women (the daughters of men) and fallen angels (the sons of God). This was the view of early Jewish scholars and likewise was held by the Christian Church until around the 4th century AD (at that time other views were put forth). Those who hold to this view believe that the fallen angels spoken of in (2 Peter 2:4; Jude 1:6) are the same fallen angels spoken of in (Genesis 6:1-4). In coming to a conclusion, it's important to remember that angels are not able to procreate (reproduce). If the sons of God are referring to angels, then what it's referring to is demon possession (not angels taking on the form of man and having children). Demons are fallen angels.

There is also the belief that the "sons of God" mentioned in the above verses is referring to the Godly line of Seth, whereas, the "daughters of men" is referring to the ungodly line of Cain (Cain being the son of Adam and Eve who killed his brother Abel).