Genesis 6:2

that the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. The Cainites had long since forsaken the Lord and His worship and lived according to the lusts of their mind. But in the course of time this corruption spread also to the families of the pious, to the sons of God, to the believers, showing itself first of all in a laxity of morals. In fifteen centuries a most remarkable increase of the human family took place, and it became increasingly difficult to maintain the discipline which the Lord desired. The men belonging to the tribe of Seth permitted carnal considerations to influence them in the choice of their wives. The daughters of men, those that lived only for this world and made the enjoyment of all that this world offers their end and goal, very naturally made the development of mere physical beauty their aim. This beauty proved the snare which captivated the sons of God, the men from the generation of believers. They took wives to themselves of all that they chose, no longer as pious helpmates in a marriage in chastity and honor, but for the mere gratification of their sensual desires. Thus the corruption of the Cainites was brought into the Church of God before the Flood.