Right, sons of God here does refer to fallen angels in their attempt to totally and utterly corrupt mankind (still their goal). Puts a little more realistic light on the occult and how we need to keep ourselves from it. They were judged by God for this (verse 7), see Jude in NT. 

Here are a couple of options for you:
The phrase "sons of God" here has been interpreted to refer ither to angels or to human beings. In such places as Job 1:6,2:1 it refers to angels, and perhaps also in Ps 29:1 (where it is translated "mighty ones").
Others maintain that intermarriage and cohabitation between angels and human beings are surley excluded by the very nature of the created order.
"Sons of God" possibly refers to godly men, and "daughters of men" to sinful women. Probably from the wicked line of cain. vv1-2 describe the intermarriage of the Sethites ("sons of God") of ch 5 with the Cainites ("daughters of men") of ch 4 indicating a break down in the seperation of the two groups.
I guess I have to believe the latter.
Does this help? 

I've studied the verses where that occurs.

The expression 'Sons of God', is like this in the manuscripts:
'giants that were downfallen angels'

The expression 'Sons of God' occures a few times in the Bible, for example in the book of Job, & always means that.

Genesis talks about giants taking doughters of humans. That was Satans plan to spoil the humankind so that Jesus could not be born from a human woman. But the plan sure failed. God threw all those giants to hell ages ago, that is also mentioned in the Bible.