The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. -- Genesis 6:2
There seems to be no good answer to this. Most believers say the "sons of God" are angels, and that these angels came down from heaven and had sex with women. But Christian Identity adherents say the "sons of God" were Adamic white men who were seduced by pre-Adamic non-white women. This race mixing event was what caused God to send the flood of Noah.