Hi Anne and Diana,
This is a curious topic, so I looked into it.
I found in 'Keil and Delitzsch Commentary on the OT' quite a long article on this (you can get this commentary from e-Sword).
Here is a small excerpt from that article.
If in the olden time there were pious men who, like Enoch and Noah, walked with Elohim, or who, even if they did not stand in this close priestly relation to God, made the divine image a reality through their piety and fear of God, then there were sons (children) of God, for whom the only correct appellation was “sons of Elohim,” since sonship to Jehovah was introduced with the call of Israel, so that it could only have been proleptically that the children of God in the old world could be called “sons of Jehovah.” But if it be still argued, that in mere prose the term “sons of God” could not have been applied to children of God, or pious men, this would be equally applicable to “sons of Jehovah.” On the other hand, there is this objection to our applying it to angels, that the pious, who walked with God and called upon the name of the Lord, had been mentioned just before, whereas no allusion had been made to angels, not even to their creation.
These 'sons of God/Elohim' seem to be talking about people that lived a moral life and were seeking to serve and obey God. They could not be considered among the elect tho, because Christ had not come as the Saviour yet.
Now the daughters of men, seem to be ungodly people, and to marry among them was seen as being unequally yolked.
Here is another excerpt.
"that the connection of Gen 6:1-8 with Gen 4 necessitates the assumption, that such intermarriages (of the Sethite and Cainite families) did take place about the time of the flood (cf. Mat 24:38; Luk 17:27); and the prohibition of mixed marriages under the law (Exo 34:16; cf. Gen 27:46; Gen 28:1.) also favours the same idea.” But this “assumption” is placed beyond all doubt, by what is here related of the sons of God. In Gen 6:2 it is stated that “the sons of God saw the daughters of men, that they were fair; and they took them wives of all which they chose,” i.e., of any with whose beauty they were charmed; and these wives bare children to them (Gen 6:4). Now אשּׁה לקח (to take a wife) is a standing expression throughout the whole of the Old Testament for the marriage relation established by God at the creation, and is never applied to πορνεία, or the simple act of physical connection. This is quite sufficient of itself to exclude any reference to angels. For Christ Himself distinctly states that the angels cannot marry (Mat 22:30; Mar 12:25; cf. Luk 20:34.).
Gen 6:4
“The Nephilim were on 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: these are the heroes (הגּבּרים) who from the olden time (מעולם, as in Psa 25:6; 1Sa 27:08) are the men of name” (i.e., noted, renowned or notorious men). נפילים, from נפל to fall upon (Job 1:15; Jos 11:7), signifies the invaders (ἐπιπίπτοντες Aq., βιαῖοι Sym.). Luther gives the correct meaning, “tyrants:” they were called Nephilim because they fell upon the people and oppressed them.
Hope this helps, it gave me a clearer understanding of this subject.
mercy, peace, and love
Kat