This is how it is: Many who study and proclaim, in speech or writing, about the days of Noah, the Book of Enoch, the Flood, and so on, claim, when they get into what Jesus said about the age of Noah, that then people would get married and be given in marriage, etc, that this is meant to depict the angelic copulation with the daughters of men.
Nothing could be more wrong. With such interpretations, they make Jesus look unknowing. Or to be very unclear in his speech. Jesus knows what he says, this was about humans, only, who would live as humans do normally, they marry and have children, everything going on as usual in the human society, and then suddenly, they get struck by judgment, just as also Paul says in his cross reference.
- But then you deny this with the angels and the giants, and the whole thingy - somebody says. Not so. Check closer in the texts, including the Book of Enoch, and the whole situation, and the picture, will be presented to you.
BEFORE the age of Noah, a whole lot of exciting things were going on, exciting to us perhaps, but terrifying to them that lived then. The angelic descent did not happen in the days of Noah, as you surely know by now, but in the days of Jared, and that is about 1100 years before the Flood, and several hundred years before Noah was born.
In the Book of Enoch and particularly in the Greek fragments that are preserved of it, it is revealed that God's judgment on the angels and their attempt, after their sons the giants were born and starting to ravage and destroy on earth - was that they were to be led out into war against each other. God judged the giants in that way, that they got to destroy themselves. This Gigantomachia, is also reflected in the Greek myths, and surely also in other ancient myths, as those from India, and others.
After this, or these wars, the turn came to the angels, for their judgment. The angels of God would go out and imprison them, after them having to see their sons die, which they had hoped so much upon, and then these angels would be buried, probably still alive, but by then since long deprived of their everlasting lives and glorifyed bodies, in grave mounds of earth and stone.
So then, when we arrive to the century just before the Great Flood, how are things on earth? Well, then it's calm! Then the giants are gone, exterminated, by themselves, and the sinful angels are gone from the earth, now dead, buried, and mouldering in their grave cairns - and the humans, who now should have thanked God for this clean-up and rescue, can again live and have dominion over the earth, as they were promised earlier.
But then this never happens. They continue to sin, and love violence and to go their own ways. This is what "grieves the Lord". They could have turned around, and even if the Flood perhaps was unrevocable, because of the whole creation having been contaminated, genetically, including the fauna, many more could have been saved with Noah on the Ark.
So Jesus knows what he talks about, when he says that it will be very calm and normal, on earth, among humans, when the Tribulation, the great, begins - first with the rapture of the Assembly, and then with the sudden destruction - just as in the days of Noah.
Because look here - had giants and descended angels been ravaging on earth when Noah built the Ark, then it had been run over and filled to the brim by people. There wouldn't have been place for an ant. But now it was calm, and they wondered what Noah was afraid of. - Now the wars and the destruction is over, Noah, we have had peace now for a hundred years! Can't you see that God is with us, and blesses our lifestyle!
No, God was just slow to anger, as Peter writes. He waited for Noah to be ready with the Ark. Ready to enter it, and leave the world, be lifted up, above it. So we also wait, and believe on what Jesus is saying, that the condition of the world will be normal, as normal as it can be in this world, when Jesus is coming to lift his Assembly, up and over the world. Here will be no tribulation going on then, just as little as it did when Noah built upon his Ark. As said, had it been so, people would have understood what he was doing, but now it was calm, and "peace and no danger", and humanity was at the mercy of their own conscience, instead of having to repent "under the gallows". Conscience is enough for you to know that you should repent, seek salvation, forgiveness, peace with God, peace with people.
Here is a reference to the Greek fragments of the Book of Enoch I mentioned, from the notes in Laurence's translation (Enoch 7:2):
"The Greek texts vary considerably from the Ethiopic text here. One Greek manuscript adds to this section, "And they [the women] bore to them [the Watchers] three races–first, the great giants. The giants brought forth [some say "slew"] the Naphelim, and the Naphelim brought forth [or "slew"] the Elioud. And they existed, increasing in power according to their greatness."
And this is the place in the Book of Enoch, where God proclaims the judgment on the angels and their sons. The speech is part of a command to the righteous angels to first go to Noah, the son of Lamech, and reveal to him the coming end and the water flood. This then is a parallel to when Noah gets to know about the Great Flood in Gen. 6, and that happened, we know, a hundred years before the Flood. So what follows here takes place just before the last century before the Great Flood:
"To Gabriel also the Lord said, Go to the biters, to the reprobates, to the children of fornication; and destroy the children of fornication, the offspring of the Watchers, from among men; bring them forth, and excite them one against another. Let them perish by mutual slaughter; for length of days shall not be theirs.
They shall all entreat you, but their fathers shall not obtain their wishes respecting them; for they shall hope for eternal life, and that they may live, each of them, five hundred years.
To Michael likewise the Lord said, Go and announce his crime to Samyaza, and to the others who are with him, who have been associated with women, that they might be polluted with all their impurity.
And when all their sons shall be slain, when they shall see the perdition of their beloved, bind them for seventy generations underneath the earth, even to the day of judgment, and of consummation, until the judgment, the effect of which will last for ever, be completed." (Enoch 10:9-12)
Gigantomachy Relief, Pergamon Altar - Pergamon Museum, Berlin, Germany