There are numerous disputes about the translation of this passage. "Nephilim" is often translated as "giants." I note now that we are about to see everything and everybody exterminated in the flood except for five humans and two of every kind of animal. (Next week!) However, the nephilim will survive, as they turn up again in Numbers 13 -- no surprise given the interpolation "and also afterward."6 When human beings began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 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.”4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
Then there is the question of who are the Sons of God. According to Christian theology, of course, there is only one, and it ain't these guys. Although the Gospel of John uses the term loosely to refer to Christians, obviously that isn't any help here. In Job, Satan and his friends are the sons of God; and they were also present at the creation. "Whereupon are the foundations thereof fastened? or who laid the corner stone thereof; When the morning stars sang together, and all the sons of God shouted for joy?" It is unclear whether the nephilim are the beings who had sex with the daughters of humans, or their offspring; and whether they are congruent with the sons of God or whether there were two kinds of alien beings banging our daughters.
It is also unclear why God is "contending" (in other translations "striving") with humans, and why limiting their lifespan to 120 years relieves this. (Again, this seems to approximately double the reality, which suggests some sort of error in transliterating numerals.) This sentiment seems to be inspired by his sons having sex with human women, but it is entirely obscure why he should feel this way.
This passage has tied theologians in knots. I won't go into it at any length since it's obviously complete bullshit, but one example is the so-called pre-Adamite interpretation. Since there are two creation stories, the pre-Adamites figure, let's go with that and assume that after the first creation, God allowed some time for the humans to multiply and establish a population before making Adam and Eve. That explains where Caine got his wife and other problems, although it creates many problems they do their best to ignore. For example, why is the pre-Adamite population cursed for Adam's fall? Did they know good from evil? Were they already mortal? And why create Adam and Eve? There doesn't seem to be any further distinction between their descendants and the pre-Adamites. The whole thing seems gratuitous.
In any case, many pre-Adamites argued that the sons of God were white pre-Adamites who were having sex with non-white descendants of Caine, and this was the nature of the "wickedness" the heart of the upcoming story we will get to next week. Seriously. If you buy that, Europeans aren't descendants of Adam and Eve at all. But they must be, because Noah and his sons are descended from Adam and Eve through Seth, and the pre-Adamites must be extinct, no?
This is theology, folks. This is what you get for studying the Bible while trying to believe in it.