For Genesis 6, the views that take the sons of God to be angels or humans both have merit to them. On one hand, there are plently of references to 'sons of God' being taken to mean 'divine beings' (Deut 32:8/43, Job 1:6, 2:1, 38:7, Psalm 29:1, 89:6). On the other hand, you can find references to humans being sons of God. Both Israel as a whole and her king, with specific references to David and Solomon, are called God's son (Ex 4:22, Deut 32, Hos 11:1, 2 Sam 7:14 [and it's parallels in the Chronicles], Psalm 2, 89:26).
So simply pointing to the phrase doesn't seem to do much other than to show a wide range of possible meanings for Genesis 6. It certainly wouldn't be wise to make an arbitrary conclusion at this point without further reasons.
In order to say that Gen 6 is talking about divine beings mating with human women, you could take the approach of comparing Genesis 6 with similiar stories in the ancient world (ie, the Greeks, Ugaritic writings, Egyptian, Sumerian, Assyrian, Babylonian, etc..). Finding parallels with other writings might be suggestive that Gen 6 is written in a similiar way. Ideas and views would be held in common across cultures. One way to aid in this would be to view Genesis 6 as originating from a seperate source (oral or written) that is no longer available to us. As such the author of this section of text would have meant what so many similiar views in the ancient world did - divine beings came down from the heavens and mated with human women generating legendary offspring.
While this is all interesting, there all a couple of downsides to all of this. One would be that even in other cultures' writings, "son of God" is also used of human beings, so you could use parallels from other writings to suppor the idea that sons of god in Gen 6 are human. Second, we must ask WHY someone would include such a thing in Genesis as a whole as we now know it. Within Genesis as we now have it and know it, does chapter 6 serve the understanding that it's refering to divine beings or to human beings? I suggest that within the literary context of Genesis, chapter 6 functions to serve to give us a reason for bringing the flood and that reason has to do with HUMAN failure, not angelic failure. You could consider a theme that continues to run through Genesis: God chooses someone, that someone fails, God judges them and chooses another: Adam, Cain, Noah. This theme runs up until Abraham when God finally chooses someone who is going to succeed due to a particular covenant God enters into with him. The theme running through Genesis of choice-failure-judgment-rechoosing fits very well with the idea that the line of Seth was chosen by God but failed. So the story in Genesis would run: Adam (chosen, failed, judged), Cain (chosen [firstborn], failed, judged), Seth (chosen, failure, judged), Noah (chosen, failed, judged), and finally to Abraham who was chosen and unlike the others, his descendents would succeed in completing God's plan (which for Abraham was that his descendents would be a blessing to the earth).
Within this literary context, the 'sons of God' in Genesis 6 would mean "chosen ones" (as it did for Israel and the king) and it would be refering to another line of humanity, and/or even kings, that had been chosen and finally failed. The purpose of course would be to lead up to Abraham and God's selection of a people that would succeed in being the human line that doesn't fail. One further point of consideration is that each of the failed human lines in Genesis (Adam, Cain, Seth, and Noah) each seem to break Torah commandments: Adam eats unclean food, Cain murders his brother, the line of Seth intermarries with outsiders (or takes too many wives), and Noah becomes intoxicated and naked. Abraham and his descendents however are given the blessing of being directly given the Torah, which at this point in Genesis looks forward to the highlight of the giving of the Law at Sinai. The effect of the literary context of the first part of the books of Moses is almost like going up a mountain with the peak being the giving of the Law at the top of Sinai. Everything in the text seems to be continuously funneled down to the point of Moses on top of Sinai. The effect of the literary context of Genesis then is to say, 'Israel, this is your God and this is why Israel was chosen'.
So, on one hand the human-sons-of-God interpretation has the literary context in it's favor. On the other hand the angelic-sons-of-God interpretation has source criticism in it's favor. In fact, I think appealing to source critical methods (something most evangelicals will cringe at) is the only way to wind up at an angelic-sons-of-God interpretation of Gen 6, and even then the literary context of Genesis doesn't just go away. If you buy into source criticism, someone has still purposefully arranged the text into the form we have it.
Another thing people like to do is appeal to the Books of Enoch and Jubilees. Unfortunately, these aren't much help since they simply reflect how some Jews of a particular sect (most probably Essenes vice Pharisees or Sadducees) of the mid to late Second Temple period thought of it. Neither Enoch or Jubilees are more ancient than about the time of the revolt of the Maccabees and so neither necessarily reflect how we should actually understand Genesis 6. Both of those books are just some Second Temple Jews trying to understand what Genesis 6 said; neither are giving us information somehow long lost from Genesis.