it's good to look at things with a fresh approach...
The angels that came to Earth could not rid themselves of all of their spiritual attributes that they were created with. They could not alter their internal makeup which is why their children were giants. This is in stark contrast to Jesus, who had a prehuman existence in heaven but was born completely human.
Perhaps this is why God said at Genesis 6:3, "My Spirit will not remain with mankind forever, because they are corrupt. Their days will be 120 years."
By coming down to Earth and mating with women the angels violated God's law of marriage, that a man would leave his father and his mother and he would stick to his wife and become one flesh. In a sense, the materialized angels who married and coupulated with women stole what rightfully belonged to man in the normal course of events.
Violating the one flesh standard was unacceptable to God, this is why the human line became contaminated and polluted by Satan's way of thining. Think of it this way. Leviticus 19:19 says "You must not crossbreed two different kinds of your livestock, sow your fields with two kinds of seed, or put on a garment made of two kinds of material." Those angels that came to Earth crossbred two different kinds of flesh; that of the spirit realm and a human which produced the Nephilim. The Mosaic law provided penalties for cross breeding two different spieces. Sexual perversions resulted in the death penalty. The angels left their natural home and came to earth. They were created in God's image and had both male and female attributes but looked upon mankind with curiosity and cultivated a desire from within their mind and figurative heart.
At the time of the flood, the angels that materialized had to revert back to their spiritual form in order to keep living. They gambled that by dematerializing, God would allow them back into the spirit realm, but once you fall from perfection there is no going back in God's eyes, so they were punished.