Good Day louisfourie
I am no Biblical scholar but I seem to recall something.... Doesn't that verse refer to the 'fallen angels' those angels that also broke with Heaven but did not exactly side with Lucifer/Satan?
I seem to recall that these angels cohabitated with human women and their children became 'giants.' The other question that arises is: Why? This bit of information does seem to be out of phase with the general narrative there.
But remember, the Bible is in many ways a patchwork job. But I can say that this kind of immortal/mortal cross-fertilization is a common feature of all mythology. I suppose it is a literary/psychological device to make human beings feel connected with the divine.
Who knows, perhaps this device paved the way in the Christian community, for the acceptance of the idea of the 'Virgin' birth of Jesus through Mary Madgedelen? I hope you get your question answered more fully, to your satisfaction.