I overlooked Naamah; my apologies.
I think your case is reasonable as far as "the daughters of Cain" argument goes. I don't think it's airtight, but I don't think it's without merit either. However, it does nothing to account for the presence of giants though. Any solution must account for all the evidence, not just some of it.
Fair enough. (no pun intended

) Lets look at that text as well.
1And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, 2That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. 3And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
This seems to be a complete thought. The Lord was grieved because the sons of God (Adams/Seths Tribe) marrying the daughters of men (Cains Tribe) would result in total wickedness so he sentences them to 120 years left before the flood.
4There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
This states that there were giants in the land in those days.
And, also (means in addition to this fact) when the sons of God and the daughters of men had children those children became mighty men which were of old men of renown. It does not say those children became giants.
The giants were in the land before the sons of God had children with the daughters of men.
How many giants? The natural reading of the statement leads one to think that it was not an uncommon thing.
"There were giants in the land in those days" (as in, not like today where there aren't and if there are it is extremely rare) What is the point?
It is not related to the offspring of the sons of God and daughters of men and I don't understand why anyone tries to force the text to say that the giants were the result of the offspring of the sons of God and the daughters of men.
It was clearly stated that they were in the land before.
And because Nephilim is a Hebrew word translated giants in Numbers 13:33 (the spies report of the sons of Anak) and Moses wrote both books, I think that would be strong evidence that he means that there were
those kinds of giants in the land in those days, 9 to 11 feet tall men like Goliath, Sons of Anak, Og, and a few other mentions in the OT.
I have read that it could be a statement about giant animals, giant plants, and giant men in comparison to those at the time Genesis was written. Is this a possible reference to what we know from the fossil records about giant dragon flies, giant beavers, giant animals, dinosaurs, and also giant men? Maybe, but because of the use of the word
Nephilim in Numbers 13:33 I lean toward it being a reference toward humans like those referenced in Numbers 13:33 where the same word is used.
I could be wrong, but being cautions to use only the most natural reading of the text I believe I have a higher chance of being correct in this interpretation than all of the other guesses I have heard presented. I suppose there is less guessing involved in this interpretation though I concede it requires the "best guess" due to lack of more information.
The question is always "Why is it recorded?" It is not for nothing.
Since so little is said then only what CAN be known should be the focus.
1) There were giants in the land in those days.
2) The sons of God and the daughters of men had offspring that became mighty men, men of renown.
3) God says that His Spirit will not always strive with men and that they will have 120 years.
Then the what becomes of the people after these ungodly marriages that were considered a striving against the Spirit of God.
5And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
And further we have this description:
The earth also was corrupt before God, and the earth was filled with violence. 12And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
Now, to me the natural reading of the text is telling me that this was what happened after the sons of God married the daughters of men and their offspring grew up and became mighty men men of renown. They also became very wicked and every imagination of the thoughts of their heart was evil continually. Corrupt here means moral decay not that of changing their DNA as some have wildly guessed. It was about sexual sins and no restraint to do evil.
It is very possible that the fact that some of them were giants made them stronger than others and they used this to their advantage to inflict violence and rapine upon others. This might be the reason it is mentioned.
If anything the reference to the giants in addition to the whole earth filled with violence paints a picture of a very frightening place. One that the readers could understand in the light of the wicked people they would be called to destroy in the land of Canaan which included some giants. Imagine the whole land filled with these instead of a small group in a tribe.
So I am getting the authorial intent about the giants as a way of emphasizing the frightening aspect of such a world given over to violence. Bad enough that they were so evil but to be so evil and also be giants would result in much more human suffering upon their victims.
I do not think that the text supports that these giants did not exist UNTIL the sons of God had children with the daughters of men. The text suggests that they were existing BEFORE these children. Therefore it there is ZERO proof that these giants were a result of this union. And that theory is built upon a rearranging of the order of the statements in the text.