And of course, the backstory provided by the extra-biblical books, like 1 Enoch and Jubilees, adds further detail—describing the Watchers, their descent to Earth, their union with human women, and the birth of the Nephilim. These texts expand on Genesis 6 and reflect how it was understood in ancient Jewish world.
As an aside, fragments of both 1 Enoch and Jubilees were found among the Dead Sea Scrolls, showing that these interpretations were taken seriously by ancient Jewish communities well before the New Testament era.
For the Angel/man theory to work, angels and men must be close enough together biologically for it to work.