God's master plan for planet Earth called for a theocracy. Man was to govern the earth under divine direction. But from the beginning, Satan has tried, at ever turn, to subvert God's program and exalt himself as king of the earth. Men commit sins because they are born with a sin nature. In our fallen state, sinning is doing what comes naturally. The union of the 'sons of God' and the 'daughters of men' was an attempt by Satan, through fallen angelic beings, to corrupt the bloodline of humanity through a progeny, part angelic and part human.