Refreshing attitude. Sincerity is a necessity when it comes to seeking the intended interpretation. I would point first to the hermeneutic rule of immediate context. Which is also the more natural form of reading comprehension whether one has ever heard of hermeneutics or not.
When you read the statement the sons of god saw that the daughters of men were fair.. .and you notice no explanation is given as to who these sons of god are, your first natural reaction is to wonder if you missed something that was already said about the sons of god. So you look backwards in the text and find in it 4:26,. Calling themselves By the name of the Lord is an acceptable translation which is found in your KJV marginal notes.
If one calls themselves By the name of the Lord, another form of that would be calling themselves sons of God or Children of God, or the People of God in some manner or fashion. They found it necessary to identify themselves as sons of God because there was another civilization on the East of Eden under Cain that were not godly. This is the most natural interpretation that one could achieve without human assistant, if all they had was a bible and no previous teaching on the subject.
Give someone a bible and a 10th grade reading comprehension skill and put them on a desert Island and the idea of fallen angels will never enter their minds but they are likely to discover 4:26 when attempting to see where they missed the reference to the sons of God in 6.
Now leaving the immediate context approach and moving on to the theological context (other scriptures in the bible that support the theological idea) If it is true that God declares his plan to bring a flood right after the statement that the sons of god saw the daughters of men and married all which they chose, that the godly fell away to the ungodly until their was not a righteous man left of the camp called the sons of God but Noah alone, this theological concept of the godly falling away by the corruption of the influence of the world is a theme that continues throughout the bible all the way to Jesus words to the 7 churches in revelation.
This is a very strong theme in the bible. The counsel of Balaam who taught Balak to send the Moabite women (prostitutes) into the camp of Israel which caused God to judge them with a plague. The story of Sampson and Delilah, Pretty much every time Israel backslid into idolatry involved harlotry, Solomon's backsliding from his foreign wives and concubines, again and again we see the lesson, into the New Testament we hear the continuous call to Come out from among her (spiritual Babylon, worldly harlotry) and be separate. The letter to the 7 churches, the call to repentance throughout the book of Revelation calling out a people who would be separate and not bow the knee to the image or take the mark or drink of the cup of the harlot. These are all symbolic of holiness and separation from the world.
Another method of interpretation is to see if this verse of scripture Gen 6:2 concerning the sons of god are referenced by any other writer in another book of the bible and commented on. I believe Jesus is indeed commenting on the sons of god taking wives of the all which they chose from the daughters of men when he said when he comes again it will be as in the days of Noah they will be marrying and giving in marriage. Jesus is referring to people and not fallen angels and therefore if he is referencing this verse in Gen then he is commenting on it as people and we have his interpretation to stand on and this should end all controversy on the matter.
Also Jesus said angels neither marry or are given in marriage, and to suggest that he meant only angels who had not fallen has no authority and seems to be a desperate attempt to cling to a erroneous interpretation thoroughly defeated.