Ge 6:1 Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them,
2 that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose.
3 And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years."
4 There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown.
5 Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually.
6 And the LORD was sorry that He had made man on the earth, and He was grieved in His heart.
7 So the LORD said, "I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them."
I stick with the traditional interpretation here. The "sons of God" referred to here are fallen angels who intermarried with the daughters of men. It is true, as said in Matthew, that angels in heaven do not marry. Of course, the angels in heaven are good angels who do not rebel against God's boundaries. We know from other portions in the Bible that angels can take human form.
The result of the union were hybrid human beings, giants. This incident here may be the source of the demigods we often read in mythology, wherein we see human-gods like Hercules, Thor and many others play a part. Aside from the genetic pollution, there was also great wickedness and depravity among humans, whom God had to punish and destroy.
This is Moses showing his Israelite readers about Satan's attempt to pollute the human gene pool to prevent the coming of the promised Seed in Genesis 3:15 who would fatally wound Satan.
Genesis 3:15 says that the Seed would come through a woman, so Satan and his fallen angels cohabited with women.
