The book of Enoch

The book of Enoch existed centuries before the birth of Christ and is still considered by many to be more Christian in its theology than the Jewish one. It was considered scripture by many early Christians. The older literature of the so-called "Fathers of the Church" is full of references to this mysterious book. The first "Letter of Barnabas" of the 2nd century makes much use of Enoch's book. The Fathers of the Church of the Second and Third Centuries, such as Justinian the Martyr, the Ireneus, the Originator and the Clement of Alexandria, make use of the Book of Enoch. Tertullian (160-230 AD) still called the Book of Enoch "the Bible". The Ethiopian Church added Enoch's book to its official canon. He was well known and read the first three centuries after Christ. This and many other books became unreliable after the Council of Justice.
This explains the origins of demons, UFOs, fallen angels and other mysteries, which is why it has been banned from previous generations. Having said that, this book was not intended for previous generations, but for our generation today.

Controversies

Some argue that the Sons of God could not be angels because:

A major theme of the Old Testament is the negative outcome of believers intermarrying with unbelievers.
Angels are spiritual beings, and therefore not reproductively compatible with human women.
Advocates of the Angel-hybrid position hold that:

Angels are recorded on two occasions in Scripture as eating food, therefore they have some ability to interact with the material world.
Angels can at times be indistinguishable from human beings, and "some have entertained angels unawares," (Hebrews 13:2)
The sexual immorality of Sodom and Gommorah is described as "likewise" in comparison to the actions of the angels that "left their proper dwelling" and that Jesus has chained in darkness until Judgment(Jude 6 & 7).
We really don't know what angels can and cannot do, as we lack the ability to capture and study them, and Scripture says little about them.
The question should be decided by the context and evidence rather than a preconceived idea about the nature of angels.
The Augustinian position fails to explain the production of gigantic offspring from the union of believers and unbelievers.
The unanimous position of Jewish and heathen authors prior to Philo of Alexandria is that the angels came down and sired children with women.