It is my understanding that Jesus (alive by the Spirit) preached to the mankind that died (spirits in prison) in the flood to redeem them. Whether or not angels took women in ancient times and produced a race of giants.Good scriptures and response.
God indeed destroyed man, as well as the giants who the the offspring of the sons of God and the daughters of men.
Notice in verse 4 -
There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown.
The children of the sons of God were giants.
The sons of God took wives and produced giants after the flood.
All died in the flood except Noah and his household.
So Noah, with his sons, his wife, and his sons' wives, went into the ark because of the waters of the flood. Genesis 7:7
The sons of God continued after the flood to produce giant offspring, when they would come into the daughters of men, and they would bare children to them.
The sons of God were not human, for all flesh perished in the flood.
The new Testament writers refer to theses as angels.
The Old Testament refers to the sons of God as angels.
The spirits in Tartarus were in fact the angels who rebelled in the days of Noah and are referred to as the sons of God.
JLB
1peter 4:6
For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.