I personally lean toward the "sons of God" in Genesis 6 being angels which kept not their first estate (Jude 1:6).
As others have written, perhaps Satan was trying to corrupt the "seed of Eve", so as to prevent Jesus from coming. I think it is pretty safe to assume that Noah and his three sons were not of this corrupted seed. But perhaps the wife of Ham was of this corrupted seed. From Ham we have;
Gen 10: 6 And the sons of Ham; Cush, and Mizraim, and Phut, and Canaan.
7 And the sons of Cush; Seba, and Havilah, and Sabtah, and Raamah, and Sabtecha: and the sons of Raamah; Sheba, and Dedan.
8 And Cush begat Nimrod: he began to be a mighty one in the earth.
9 He was a mighty hunter before the LORD: wherefore it is said, Even as Nimrod the mighty hunter before the LORD.
10 And the beginning of his kingdom was Babel, and Erech, and Accad, and Calneh, in the land of Shinar.
11 Out of that land went forth Asshur, and builded Nineveh, and the city Rehoboth, and Calah,
12 And Resen between Nineveh and Calah: the same is a great city.
13 And Mizraim begat Ludim, and Anamim, and Lehabim, and Naphtuhim,
14 And Pathrusim, and Casluhim, (out of whom came Philistim,) and Caphtorim.
15 And Canaan begat Sidon his firstborn, and Heth,
16 And the Jebusite, and the Amorite, and the Girgasite,
17 And the Hivite, and the Arkite, and the Sinite,
18 And the Arvadite, and the Zemarite, and the Hamathite: and afterward were the families of the Canaanites spread abroad.
19 And the border of the Canaanites was from Sidon, as thou comest to Gerar, unto Gaza; as thou goest, unto Sodom, and Gomorrah, and Admah, and Zeboim, even unto Lasha.
20 These are the sons of Ham, after their families, after their tongues, in their countries, and in their nations.
I think it was Bro Parrish who sometime ago showed that Ruckman teaches that those tribes with "im" on the end were giants. Here we have Mizraim, Ludim, Anamim, Lehabim, Naphtuhim, Pathrusim, Casluhim, Philistim, and Caphtorim. I think it is significant that Nimrod also has "im" in his name.
I think it is also significant that Sodom and Gomorrah are mentioned in vs. 19, cities known for their wickedness, especially fornication and going after strange flesh.
I have posted this before, but here is a site that claims that Sodom and Gomorrah have been found. You can find several sites with information on this, but this is a good one with photos.
http://www.arkdiscovery.com/sodom_&_gomorrah.htm
I don't know if this is the specific site, but awhile back I did a study on this, and it was written that the doorways and rooms found here were very large, tending to imply that the residents of these cities were much larger than ordinary people, in fact, giants. I will try to find that site and post here.
And of course we know of giants such as Goliath, so giants persisted some time after Genesis.