The passage you refer to is Gen:6. When satan rebelled against God and left Heaven, he took a third of the angles, ie: " sons of God ", with him.
These fallen angles took human wives, " the daughters of men ", and from this unholy union where born races of giants ,the nephilim. This is where we got giants such as Goliath and others from. Satan's plan ,it seems was to desrtoy the human race by crossing these supernatual beings with humans thereby leaving no pure strain of humans left.
It almost worked because by the time of Noah, he was the only human found to be " pure in his generations ", left on earth. This did not mean he was sinless, but that his family tree, going back to Adam was the only one that had not been inbred and corrupted by these fallen angles. This was the reason God sent the flood, wiped them out and started over.