Genesis 6:1-4 says:
'When men began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of men were beautiful, and they married any of them they chose. Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
The Nephilim were on the earth in those days-and also afterward-when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown.'

It's just before Noah builds the ark - does it mean that the men aren't fully human but the girls are? Is it just a term that was used? Does it mean something else entirely? If so, why are the men Sons of God but the women are daughters of men? And why did their offspring become the heroes of legend? It sounds like the stuff of Greek mythology.

If I've read it before then I didn't register it - and I was so surprised by it that I went to find the NIV to see if it said the same. (The one quoted above is NIV, so yes it did.) I'm reading through the Bilble chronologically in a year so this will probably be the first of many queries I have - what does it mean?

Any ideas?