Note: Many years ago I attended a Bible study on the book of Genesis. The pastor who was leading the Bible study said he wanted to go through every chapter in Genesis - but what really happened was this: We covered Genesis 1-5 in a very short period of time. And then we got stuck on Genesis 6, specifically, verse 2 which says,
"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
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