David Mills picked up Aaron Denlinger's take on Calvin's interpretation on this as the sons of God being the children of Abel and the sons of men being the children of Cain. In other words, this is about mixed marriages with those who do not believe.
When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. Then the LORD said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown.
The LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. And the LORD regretted that he had made man on the earth, and it grieved him to his heart. So the LORD said, “I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them.” But Noah found favor in the eyes of the LORD. Genesis 6:1-8 ESV
Read part of it here:
Calvin adopts the view that "sons of God" is here a reference to the descendants of Seth, among whom "the pure and lawful worship of God" had thus far prevailed, while "daughters of men" refers to "the children of Cain." In part he adopts this perspective by a process of elimination: when in doubt, choose the interpretive option that doesn't entail ascribing corporeality and corporeal functions to angelic creatures or serve to absolutize a social construct like the relations of nobility to peasantry. Calvin's view also, however, has the merit of respecting its context; the chapters leading up to Gen. 6 serve to detail the genealogies and differences of Cain's and Seth's lines respectively. It is, then, most natural to read the reference to "sons of God" in Gen. 6.2 as a further reference to Seth's line, which has already been identified as proper worshipers of the true God, and so to see Gen. 6 as advancing the great drama of that conflict between the respective seeds of the woman and the serpent -- a conflict which will reach its apex in Christ's life, death, and resurrection.
The best part is his conclusion of Calvin's and his own take on Genesis 6:
Step number one for choosing a spouse, then, is this: choose a believer. Calvin's rather black and white moral exhortation, and the rather overt assumption underlying it (that single Christians are not in fact free to marry absolutely anyone they might wish), will undoubtedly prove jarring (even to some Christians) in our present day culture, which is squeamish about moral absolutes pertaining to relationships and seems particularly hell-bent on stripping away restrictions on who individuals can lawfully set their affections upon. C'est la vie, as Calvin never said. The good news for single Christians is that Calvin has no problem with you pursuing, with the intent to marry, a believing person of the opposite sex because you think that person's smoking hot (among other virtuous qualities, of course).
And therein lies the rub. Even among faithful Christian folks, faith is seldom a high priority much less a deal breaker when it comes to choosing a spouse. Added to this is choosing one who shares your own particular faith (example Lutheran), and it becomes even less of a priority or consideration. Indeed, looking out on my own congregation I see many families whose sons and daughters have either married non-believers and taken on their lack of faith or who have married non-Lutherans and their babies are not baptized and they seem no longer to miss the Eucharistic food of Christ's body and blood.
I am not writing to condemn but to suggest that the consideration of faith or its lack is not some little side issue for choosing marital prospect. It is pivotal. If your choice of spouse is not Christian, I think that must be a deal breaker for the Christian. If you choice of spouse does not mirror your faith (say Lutheran, for example), then the couple ought to come to a decision about where to raise their children and which will be the church home of their family. I have seen many marriages in which the convert (to Lutheranism) became the better member than the one raised in it. I have also seen many marriages in which the lack of shared faith contributed to the falling away of the one who did believe. Again, my point is not to blame but to encourage. Faith is important in the choice of a spouse. It ought to be a deal breaker and Christians ought to be forthcoming to their non-Christian marital prospects about this and should be intentional about seeking a spouse who can confess with them the faith in which they were raised.
Sadly I know of a few families in which the father was not even able to act as a sponsor to his own children when they were baptized in the mother's Roman Catholic Church -- even though the dad was a Lutheran pastor! Such divisions within the fabric of the family faith are hard on families. In other cases, I have seen the burden borne by the believing parent in being alone in church with his or her children and finding no support at all in the seemingly eternal battle for Sunday school and catechism attendance.
Wake up, folks. Faith counts!!