"That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose." - Genesis 6:2 (Audio)
Here are some sermons on this admittedly difficult verse:
The Sons of God in Genesis 6, by Rev. John Greer
How Churches and Societies Degenerate, by Rev. Graeme Craig
As it Was in the Days of Noah, by Rev. Gordon Dane
Days of Noah Before the Flood, Part 1, by Dr. Joel Beeke
Days of Noah Before the Flood, Part 2, by Dr. Joel Beeke