Close Look at Genesis 6:2

"But gradually the people of God forgot their high calling, loosed down their integrity and stepped down from the high estate of the sons of God and mingled themselves with the seed of men in the ways of the flesh. 

Genesis 6:2 The sons of God saw the daughters of men that they were fair; and they took themselves wives of all which they chose. Note that it was not that they took themselves wives of only such as

they chose, as if each one had taken him a wife from the daughters of men.  ...they took themselves wives of all which they chose; –they entered upon the polygamous practice of the sons of Cain. 

If they had maintained their integrity, then the sons of God taking from the daughters of men, bringing in from the descendants of Cain, each of them only a wife, would have been bad enough. 

For, such numbers of unconverted women, caring nothing for the way of God, and knowing only the wild ways of the descendants of Cain, could have had no other effect than to wipe out the clear distinction between the children of God and the children of this world....; it was the far worse thing of the sons of God crossing the line and going over into the field of the daughters of men, the field of the descendants of Cain, and adopting their polygamous practices. And the effect of this was as a mighty tide to sweep the world into the very depths." 

A.T. Jones