The fallen angels that came to Earth looked upon mankind and became interested in materializing to see what it was like to become human. They materialized as both men and women and became inflamed with lust for each other and humans because with the act of anything comes a pattern that is ingrained in the mind. They could not ignore the feeling of sex because they had already cultivated a desire for it within, just as Satan cultivated pride.
Peter describes what happened to them: "In that state He also went and made a proclamation to the spirits in prison who in the past were disobedient, when God patiently waited in the days of Noah while an ark was being prepared; in it, a few that is, eight people were saved through water.”
The fallen angels crossbred two different kinds of flesh, human and spiritual, the result being the Nephilim. The fallen angels that had God's spirit were mild tempered but mankind were degraded and violent.
Something else to consider is the birth of Jesus. Despite having a prehuman existence, he was born fully human with no trace of his previous angelic attributes whereas the fallen angels were unable or unwilling to shed their spiritual attributes. This gave their offspring superhuman strength and intelligence and bought them the fame they sought.
The situation had got so bad that God felt compelled to issue a proclamation at Genesis 6:3: "Then the LORD said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”
This message was meant for the fallen angels who took on human form. God would allow 120 more years before bringing the global deluge. This date was known to those in heaven and not those on earth because the Bible was written much later, so no one knew when the end would come.
God eventually brought an end to the long cycles of life that men enjoyed, you can see this in Genesis where each generation died earlier than the previous generation. No one lived a day in God's eyes except for the materialized fallen angels and the Nephilim who were not under the original judgement.
Genesis 6:12, 13: "God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. So God said to Noah, “I am going to put an end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth."