Genesis 6:1-5

ויראו בני האלוהים, the sons of the judges, the elite of the species who are here referred to as אלוהים, just as in Exodus 22,27 אלוהים לא תקלל, “you must not curse a judge.” There are numerous similar examples in the Torah of the attribute אלוהים being applied to morally high ranking human beings. Onkelos also translates it in this vein when he speaks of בני רברביא. The explanations offered that the Torah refers to Uzza and Uzael are very far fetched. (compare Torah shleymah on this verse). [The two aforementioned are supposed to have been angels who had voted against the creation of man and who belittled man’s effort to conquer the evil urge. They were supposedly punished by being consigned to earth to experience life on earth by themselves. Instead of proving their superiority, they were the first to succumb to the allurement of physically attractive females. Their request to return to the celestial regions was denied by G’d. Ed.]
According to Bereshit Rabbah 26,5 Rabbi Shimon ben Yochai cursed anyone who translated the words בני האלוהים as “the sons of G’d.” He added that when the elite of the people are themselves guilty of the sins they want the common man to refrain from, their efforts are doomed to start with. Another reason offered why these people were referred to as בני האלוהים, (according to both Rabbi Chaninah and Rabbi Shimon ben Lakish, is that these were the people enjoying carefree long lives, free from sickness and other impediments. [This may have given them the illusion of being able to get away with whatever they felt like doing. Ed.] Rav Hunna, quoting Rabbi Yossi, says that these people were granted such long lives in order to enable them to study the sky, observe the orbits of the stars and make calculations resulting in our calendar.

את בנות האדם, the daughters of the weaker specimen of the human race. The women were unable to resist the physically superior specimen referred to as בני האלוהים, and as a result if fancied by the latter, they would be taken by them as wives or concubines against their will, and the will of even their husbands. [The Torah describes the first time the law of the “survival of the fittest” was practiced by the human race. The result was the deluge. Ed.] The term אדם next to the term אלוהים simply compares the weak to the strong In the Sifri¸ (Behaalotcha 86) the point is made that when ordinary individuals observed that the judges took the law into their own hands, they reasoned that they themselves would most certainly be able to get away with the same thing, seeing that the judges are supposed to set a good example.
Rabbi Avraham Ibn Ezra understands the expression בני אלוהים, as referring to spiritually superior people, people who knew what G’d expected of them. They chose wives according to what they perceived was suitable for them basing themselves on their reading of astrology. As a result of such careful mating, their offspring were in many instances giants, etc., as we read about at the end of our verse. It is possible that they took women of their choice even against the will of these women.

וגם אחרי כן אשר יבואו בני האלוהים אל בנות האדם, and also after the elite had made a point of mating with the physically attractive בנות האדם there were even more numerous such giants who reflected the genes of their fathers in size and the allure of their mothers in beauty. והמה הגבורים אשר מעולם, these were the ones always remembered in later times as the heroic people of their time אשר מעולם אנשי שם, people whose reputation survived their disappearance from the earth. After the deluge survivors realised that it had been displeasing in the eyes of G’d for those people to have devoted themselves to all the physical pleasures of the world and to aim at enlarging the size of their bodies as well as aiming to increase the number of years they lived on earth. [After all they had perished, and normal sized individuals, such as Noach’s family had survived. Ed.] The physical desires of the survivors of the deluge were far less powerful than that of their forebears so that gradually also their lifespan diminished, i.e. their entire hold on matters physical became a less dominant concern. People in the generations after the deluge, in spite of their other failings, gradually resumed a lifestyle that had been normal before the excesses that the בני אלוהים had indulged in. When the Israelites came to the land of Canaan there were only a very few of the former giants still surviving, mostly concentrated around Hebron (Joshua 15,14) Our sages have a tradition that there were only four such giants still alive at that time. (Joshua 14,15) In the time of David there appear to have been a few such men in Gat (Samuel II 21,15) Concerning Og, the King of Bashan, the Bible testifies that he had been a remnant of this race of these antediluvian giants (Joshua 13,12). It is possible that those survivors still practiced a lifestyle reminiscent of their antediluvian forebears.