Sin is insidious and it spreads like a bad cold. Sometimes you can watch the disease leap from one person to another; other times it seems like it's just borne on the wind.
In Genesis 6 sin makes a leap from humankind into the cosmos in general. What's this business about the "sons of God" lusting after and marrying human women? Various interpreters have had a field day with what is actually being talked about, but it's clear there's a pretty significant boundary being crossed. When God created the universe in Genesis 1, there was a place for everything and everything in its place. Out of chaos (meaning, things go just anywhere), God created an orderly existence. But now sin has broken the boundaries and the result is a return to chaos. So here we see even the existence of orders of creation breaking down. Angelic beings have sexual relationships with humans, which was never part of God's intention.
Nearly every ancient culture in the Mediterranean world had some story about the gods producing children on human women. The Greeks had the Titans; the Babylonians had Gilgamesh. The Egyptians believed Pharaoh to be a son of the gods. Each of these cultures looked to these cosmic cross-breeds for salvation. But the Bible is saying that this kind of half-breed divinity is not enough to save you. In fact, it results from a breakdown in the boundaries of creation. You need more than a half-divine warrior to rescue you from the powers of sin.
The irony is that like most counterfeits, this one is almost right. Because when God acted decisively to save this world, he engineered things so that he himself was born of a human woman. Jesus was not half divine, he was all divine. And all human. 100% of each. And rather than exalting himself as the mightiest warrior the world has ever seen, he humbled himself. His road to glory went through the cross.
So as sin expands farther and farther into the created world, breaking down relationships and throwing order into chaos, God is anticipating that someday soon he will move to create order, to heal the brokenness, to sacrifice himself for his beloved creation. In the next few verses of Genesis 6 we see the impact that sin's spreading has on God's heart -- he is in agony over it, broken by the brokenness of his beloved.