I too would be interested in an explanation of this chapter. (Any explanation that makes sense that is) Ive heard various explanations, the most popular being that ''The sons of God'' were fallen angels who mated with human females and produced Nephilim (Giants).
However...
//There were giants in the earth in those days; and also after that//
makes no sense if they were all supposedly wiped out by Noah's flood (they crop up again, post deluge, inhabiting Canaan).
The fallen angels appear to be a randy lot if this is the case.
Never received an explanation though of how exactly a spirit being can produce DNA (via sperm) and impregnate a woman!