Genesis 6:5

"The Lord saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time." NIV translation

These "sons of God" probably did not intend to turn their backs on God and they may have even thought that they could bring these women to the Lord. Instead, what happened was these women led the men away from God and into idolatry, especially what we would call today "humanism". Like Cain's descendants, these "sons of God" and their children went into a downward spiral until they were completely controlled by sin and Satan.

Genesis 6:6

"The Lord was grieved that he had made man on the earth, and his heart was filled with pain." NIV translation

Just as we mourn at the death of a loved one, God was mourning the sad state of His creation. His heart ached for the people and all of the creatures that were under man's care. Just as a parent is filled with sorrow when they see their child going down the wrong path, God was pained to watch His children not only walking down the wrong path but even running in the wrong direction.

Genesis 6:7

"So the Lord said, 'I will wipe mankind, whom I have created, from the face of the earth - men and animals, and creatures that move along the ground, and birds of the air - for I am grieved that I have made them.'" NIV translation

God is love and with the deepest of love comes the dark side of love which is jealousy (see Zephaniah). Man was created for God and He could not allow this complete disregard for Him to continue.