When Cain’s descendant Lamech took 2 wives, he opened the door for acceptance of plural marriage to his own undoing, because now that society’ accepted plural marriage, the Nephilim in all likely hood took a multitude of wives in order to build up their own armies, who in turn brought down the descendants of Cain’s descendant Lamech & everybody else, except the 8 on the Ark.
Why aren’t the sons of the Nephilim still being produced? It’s because the angels that were involved in this ultimate degradation of the family are now locked up in pits of darkness in the deepest pits of Hell, otherwise, it would still be going on! Jude 6 & 2 Peter 2:4-5 .
Noah wasn’t told to build the Ark until after Shem, Cham, & Yafet were married adults. By that time all his other descendants (if they existed) were dead! My guess is that the Nephilim killed all his other descendants in retaliation for his preaching against them.