One valid interpretation I've studied out is that the term "sons of God" isn't talking about "Seth" but the term is a title that rulers took upon themselves, sort of claiming divinity. These "sons of God" (kings claiming divinity) took wives all of which they chose and their children where the "giants". Perhaps these were rulers because of their stature and strength and their children had the same traits. I find that plausible but it still denies the interpretation held by the ancients.
Here's an interesting link to review on the angelic theory by a Oneness Pentecostal...
http://www.onenesspentecostal.com/genesis6.htm
There's so much to the spiritual realms. I don't doubt for a moment that we see an angelic fall here.