The uniform Hebrew and Christian interpretation has been that verse Ge 6:2 marks the breaking down of the separation between the godly line of Seth and the godless line of Cain
I'd hate to disagree, but this is just not true.
The Sethite interpretation is rather late, appearing first in Julius Africanus (AD 160-240) and adopted by many later Fathers, along with Luther and Calvin. The angelic interpretation however is clearly older, appearing in the LXX translation of Genesis (dating to the third century BC), the "Book of Parables" of 1 Enoch (late third century BC), the "Book of Giants" from the Dead Sea Scrolls (late third century BC), Jubilees (mid-second century BC), the Genesis Apocryphon (second century BC), the Damascus Document (second century BC), the Ages of the World, aka 4Q180 (dating to the second-first century BC), the Testaments of the Twelve Patriarchs (second century BC-first century AD), Philo of Alexandria (first century AD), the "Book of Parables" of 1 Enoch (first century AD), Josephus (first century AD), and later in Papias (early second century AD), Justin Martyr (second century AD), and Tertullian (second-third century AD). The passage in Jude 6 is clearly influenced by 1 Enoch, a book that has influenced the epistle of Jude as a whole:
http://www.jehovahs-witness.com/10/85223/1.ashx
What is more, the angelic interpretation is clearly derivative of the OT use of phrases like bny 'lhm, bny h-'lhym, and bny 'lym to refer to divine beings (cf. Deuteronomy 32:8, Job 1:6, 2:1, Psalm 29:1, 89:6), and Ugaritic usage of bn 'il and 'dt bn 'ilm to refer to the lesser gods (cf. KTU 1.4.III.14, 1.15.II.7, 11). Compare also the allusions to the kwkby 'l and kwkby bwqr in Job 38:7, Isaiah 14:13 (cf. the corresponding pchr kkbm "asssembly of the stars" in Ugaritic texts), and the reference to the fallen angels as "stars" in 1 Enoch and other kindred texts.
Finally, as has been pointed out by Skinner and others, the Sethite interpretation is not felicitious with the Hebrew wording in Genesis 6:1-3, as it would require bnwt h-'dm "daughters of men" to differ in reference between v. 2 and v. 1, h-'dm especially would shift in meaning back and forth between v. 1, v. 2, and v. 3, and there is no obvious textual reason to restrict h-'dm to Cainites.