in the early christian apocryphal scripture "the treasure cave" (and probably even earlier jewish traditions) the author tries to explain, that the nephilim were not the sons of the angels, because it's impossible for an angel to have sex. rather than that, cain's offspring mixed with seth's children, leading to a giant race.
what i find interesting about seth is, that the egyptian quasi-pendant to satan was seth (horus' brother, the god of sex and power - or as we say today "sex, drugs and rock'n'roll") while the anti-egyptian israelites had seth as their (good) forefather. i wonder if horus has some connection to cain then.