You said — “I think the flood took care of the Nephilim.”
Yes, it did... but — there are more fallen angels. And Satan’s time is coming, in which he’ll be granted power over the kingdoms of this world, more so than he’s had thus far. So, Jesus’ words, in that it will be similar to the time of Noah, indicates to me that we’re facing another outbreak of the same sin that the angels committed back then — but now, in the present, at the time of Antichrist...
Also remember that the angelic/human shenanigans started up again after the flood. IIRC this info is found in Numbers.
For a LOT of insight into this issue, read the Book of Enoch. It’s extra-biblical, but I think the book is afforded a lot of credibility by the fact that it’s quoted in the Bible, specifically in Jude.
The Book of Enoch goes into great detail on the angels who came to Earth and took women. Names of the leaders of the angel pack, their discussions, their eventual punishment, etc. Fascinating stuff.
MM