That little detail of logic destroys their argument.
Actually the very same can be said going the other way on that argument for a moment. Where did we get the races from after the flood? (Other than postulating that the tower of Babel included race when it only talked about language, which goes outside of the language far more than understanding sons of God to mean angels.)
I'm merely pointing out the ambiguity of the flood to show that we don't know all of the details. I personally witness that the Bible talks about the flood for a variety of reasons. The very obvious is the "traditional" and simple view that God flooded the world because of man's unrighteousness. Yet the Bible goes further to hint at some "deeper" things, if you will. I've seen two very good cases made for the flood being entirely global and then another for it being regional to the area of the earth where humans lived.
Jude 1:6-7 seals this together, I won't rehash what Angelina has already stated very well.
I would add to the discussion:
I Corinthians 11:7-10 NRSV
For a man ought not to have his head veiled, since he is the image and reflection of God; but woman is the reflection of man. Indeed, man was not made from woman, but woman from man. Neither was man created for the sake of woman, but woman for the sake of man. For this reason a woman ought to have a symbol of authority on her head, because of the angels.
I've seen people try to read a weird spiritual contortion into this passage, but I prefer to take it at face value. The passage does not randomly deviate from talking about the physical nature (IE: attraction) of men and women, but clearly addresses an issue where angels are also attracted. I chose the NRSV because I think the version is pretty clear about it all - even the margin notes that there is no consensus of this passage. There is no consensus, I submit, because it makes people uncomfortable.
While it was clearly setup for angels and humans to remain seperate, there are some not-so-subtle clues that this did not happen. God also setup several books worth of laws to follow, and how well did that work out for us as humans? We are told precious little about how Satan got from point A (light bearer, cherubim) to point B (devil). What little bit we are told is highly figurative and thus we don't have a crystal clear narrative of why Satan ended up in the garden to deceive.
The Bible goes out of its way to talk about adultery. There are no coincidences with the Word of God.