What do you think of the theory that Genesis references to "Daughters of Men" and the Nephilim (giants)....?

Actually it isn't scripture that confirms or denies. It is DNA. Giant-ism is caused by a genetic mutation. People in the ancient times who were considered giants were deemed so because they were taller than the rest of the population. This happens when 2 people carrying the Giant gene have a child. Several years ago they did a Mitochondrial DNA study and found they were able to track the populous of the earth to a bottle neck several thousand years ago - the flood. So technically if they did a mitochondrial DNA study and found giant-ism in the mitochondrial DNA they would be able to trace that genetic mutation back to its origins.