This extract is taken from the life of Seth in the Chronicles (sigma 295) of George the Monk (Georgius Monachus, Chronicon 44.11-16, Chronicon breve 110.88), with certain omissions: an alternative "or sons of the gods, according to Symmachus" (found at sigma 295) and the names of the unjust ancestor, Cain, and the just one, Seth.
This tale seems to originate either in an unknown passage of Symmachus (OCD(4) 1417) or in the Chronologica of Joannes Malalas (p.7.7 Dindorf, 1831), who, as usual, has "Greek mythology incorporated within a framework of Hebrew affairs" (OCD(4) 890). The ancestry of the Giants in Greek theogonies is here fitted to the Bible story of Seth, the third son of Adam and Eve (Genesis 4-5) and thus the grandson of God. In fact, Enos is there the son of Seth, and Enoch the son of Cain, a fact that is confused in the Greek sources here under discussion. Enoch is an example of virtue in the Bible. Cain has no daughters mentioned in the Bible; thus the incestuous marriage must derive from Greek mythology.
The word miaigami/a is a bizarre late compound from the same root for pollution as miasma (mu 1028) and miai/nw (web address 1), followed by the root for marriage. Outside the passages above, it is known to us only in two passages of Clemens Romanus. The only similar compounds are three words related to murder (miaifoni/a, miaifone/w, miai/fonos). Normal derivatives are in mia-, e.g. miaro/s, or mian-.
[1] Notice the (post-classical) third person plural of the aorist imperative passive of noe/omai. This imperative is used frequently in treatises on mathematics, where we might in English use "Take... as" or "Let... stand for."
[2] Editors of these texts assume that this is a false reading for a)katallh/lou 'unhallowed', which makes more sense.