“Now it came about….the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose.”  Genesis 6:1-2

This passage may cause us to envision women being taken against their will, but it does say “marriage.”  First, bear in mind that the point of the flood was not to destroy the Nephilim/Giants (and we know it did not), but was to destroy mankind on account of his wickedness.  So I must ask, who was performing these wedding ceremonies?  It does not merely say that the sons of God were intimate with the daughters of men — that could have been done without marriage.  Due to the placement of this passage in context of God’s displeasure with humanity, I can only assume that God was further upset at humankind by these marriages taking place.  In short, God is holding man accountable through these transactions involving fallen angels.  I suspect God does not expect much good behavior from them, and as such is instead holding mankind responsible for their increasing wickedness.