As each generation passed, men were moving farther and farther away from God, so that by the time of Noah and a bit more than 120 years before the Flood, Genesis 6:5 records that the wickedness of man was great on the earth, and every imagination of the thoughts of his heart was only evil continuously. It was so bad that God determined to bring a judgment down on billions existing in those days where every imagination of their thoughts of men were evil continuously. With an eye to continue moving His plan forward, He gave Noah the tools he needed by His grace to build the ark as God patiently continued to move His plan forward toward the promise of the merciful redemption He had given in the Garden.
This section of Genesis 6 has been the subject of much speculation over the years. But as Jesus Christ's own words tell us, angels were not created to have sexual relationships with men, even though we can see that it certainly seems to be clear from the biblical reference to angels that we see also in Job 1:6, 2:1, and 38:7 that these probably were angels, referred to as the sons of God.
But keep this in mind, God creates kind after kind. Angels cannot cross over that line even in disobedience to God. However, as men take themselves out from under God's protection by blatantly disobeying His laws, they put themselves at risk of demonic influence, even in some cases, demonic possession as we clearly see in the Gospels.
With this in mind here is a very clear and important warning we can glean from the scripture to be cautiously applied for us living in these last days. It is from Richard Ritenbaugh's sermon, The Days of Noah. Although given back in 2008, it is a look into today and what lies immediately ahead, because God is continuing to make sure that all men will ultimately understand that everything depends on Jesus Christ.
Brethren, it was Satan's demonic attitudes and methods that were driving men who had removed themselves far from God, that they left themselves wide open to having every thought they had to be evil continuously. It was the same demonic spirit that would eventually drive those Jesus Christ spoke to as recorded in John 8:43-47, to crucify Him, because they loved not the truth, to live it. Jesus Christ said:
It is the same demonic influence that will produce the end time Nephilim and society as in the days of Noah, so driven by the spirit of Satan and his demons that every imagination from the heart of men will be evil continuously. The only exception will be those who are faithfully learning and living the Word of God through Jesus Christ.