In Noah's time, god sent some angels down to us. They had strict instructions to remain chaste, I think that's the wording the Bible uses. I've always been curious... why would god create angels with sex drives? They must have them in order to find human women attractive.
I addressed this a long time ago. The Nephilim were the offspring of Seth's descendants and Cain's descendants.
Angels can't have sex because:
1. their spiritual bodies probably do not have the capability to have sex (sex and food are distinct (1 Cor. 6:13). Just because they can eat food doesn't mean they can have sex)
and
2. God would never allow that.
Since He considered intermarrying among His people abominable (a manifestation of His zeal for His people), I doubt that He would even think of allowing intermarrying between angels and humans.
3. Furthermore, angels don't have the ability to do whatever they desire unless granted by God. Angels can't do anything without God's authorization. This is because of obligation and fear (James 2:19). We see multiple instances of obedient fallen angels in both OT and NT.
But how can this be? Gen. 6 says there were giants. Yeah, so?
Genesis indicates that people lived long lives. This is, presumably, because of "the waters which were above the expanse". So it's possible that this benefit also allowed them to reach great heights.
Only the depraved and perverse would accept such a abominable doctrine, that is, angels having sex with women.
The proper interpretation is surely the descendants of Seth and Cain intermarrying.