For if God did not spare angels when they sinned, but sent them to hell, putting them in chains of darkness to be held for judgment;
2 Peter 2:4 (NIV)Enoch, the seventh from Adam, prophesied about them: “See, the Lord is coming with thousands upon thousands of his holy ones
Jude 1:14 (NIV)
The books of 2 Peter and Jude have passages that seems to quote from the Book of Enoch, which is not in our Bibles. While this book is not Scripture, the point is that Jude, Pete, and their contemporaries were familiar with the text.
The book of Enoch says angels had sex with human women. Giants were born in those days, and in the days afterwards.