If demons were able to breed with human women and produce Nephalim before the flood, why couldn't they after the flood?
We suggest you first read our article, How did fallen angels and humans produce Nephilim?
As the Bible says in 2 Peter 2 and in Jude the demons responsible for mating with women, a brazen act of rebellion following their exit from Heaven, were condemned to the Abyss following the flood. This served as a warning to all demons not to repeat that sin:
2Pet. 2:4 For if God did not spare angels when they sinned, but cast them into hell and committed them to pits of darkness, reserved for judgment;
2Pet. 2:5 and did not spare the ancient world, but preserved Noah, a preacher of righteousness, with seven others, when He brought a flood upon the world of the ungodly;
Jude 6 And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day
Later in the Gospels we see how much the demons dread this punishment:
Luke 8:30 And Jesus asked him, “What is your name?” And he said, “Legion”; for many demons had entered him.
Luke 8:31 They were imploring Him not to command them to go away into the abyss.
So the threat of confinement in the abyss has kept demons from repeating the same error since the flood.