Well, if the "sons of God" are those humans who belong to God, and "men" are those in the line of Cain---not chosen by God, then we have intermarriage between the godly and the ungodly. Ever heard the expression one bad apple spoils the bushel? Or how about a little leaven, leavens the whole lump.
Where do we see those who belong to God called sons/children of God?
Romans 8:14
For those who are led by the Spirit of God are the children of God.
John 1:12 Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God.
1 John3:1 See what great love the Father has lavished on us, that we should be called children of God! And that is what we are.
Romans 8:15 The Spirit you received does not make you slaves so that you live in fear again; rather, the Spriit you received brought about your adoption to sonship.
2 Cor 6:18 And, "I will be a Father to you, and you will be my sons and daughters, says the Lord Almighty.
It is far more trustworthy to go by what the Bible says, even if one cannot fully understand it, rather than to try and gain supposed understanding from someplace that God had declared "Not his word" by excluding it from his word. Such as the Book of Enoch.
What if and why not? You might consider that the original hearers knew exactly what was meant, and that is who it was given to. The fact that centuries later people began to have trouble and felt the need to speculate about it is irrelevant. I could just as easil say, "If it meant fallen angels procreated with human women and produced giants", it would have said, "Fallen angels saw that the daughters of men were desirable and had sex with them and the combination of the DNA produced giants."
However. That passage becomes spurious evidence for your view because it ignores the statement that angels do not marry (therefore do not procreate). And it ignores the design of like producing like.
You're right. It doesn't explain it. It just said that it did. In the account of Jacob and Labin and the spotted goats. Remember? It does not explain the science of the genetics either. It just says, "If you do this, that will happen." Why do you think God has to explain every thing to you in order for it to not be open to wild speculation and imagination. "Just believe!"
That doesn't mean that it was used that way in Gen 6. Just because Enoch said that it did, and just because he was the seventh from Adam does not lend it authority. If he even wrote it. Here is something for your education.
Why Is the Book of Enoch Not in the Bible?