Let me clear up a common misconception. There is nothing at all in the flood story about God sending the flood because of 'adultery' or homosexuality - this is pure speculation and nothing more. The only specific sin that is mentioned is "violence":
God said to Noah, “I am going to put an end to all people, for the earth is filled with violence because of them (Gen 6:13)
I also have to smile at the highly condescending comments of ladyren, who claims to have attended the University of Chicago divinity school but doesn't know the difference between Sumer and Samaria! Two of her claims deserve to be challenged. Firstly, the flood story in Genesis is not an exact copy of the one in the Gilgamesh epic, as she seems to infer. Although it IS clear that Genesis borrowed the story from older Mesopotamian literature, Genesis leaves out about a third of the Gilgamesh flood story that has to do with the actions of the gods, and the meaning changes dramatically. In Gilgamesh and Atrahasis, the gods err and are strongly condemned for punishing innocent humanity. In Genesis, the flood story is a parable of judgment on a wicked and violent civilization. If ladyren knew anything about the way Genesis reuses Mesopotamian myth, she would know that it it does so in polemical fashion, satirizing and subverting the underlying political and cultural messages of the older stories. The target of the Genesis flood myth is probably the Neo-Babylonian empire itself.
By the way, you are way off in your guesswork about the dating of Genesis. Most scholars date it to around the 6th century BCE and even fundamentalist Christians claim it was written by "Moses" no earlier than the 15th century BCE. The earliest physical evidence for Genesis dates to around the second century BCE (the dead sea scrolls). On the other hand we have physical copies of the Epic of Atrahasis containing the original version of the Gilgamesh flood story that date to 1700 BCE. These in turn are modeled on an even older Sumerian version (sometimes called the Eridu Genesis) that probably originates in the third millennium BCE.
Ladyren's other quite ridiculous claim that 'about [sic] all the stories in the OT are copied from other and older religions' is patently false. This is a common exaggeration in online forums that has no basis in serious scholarship. The same small handful of examples is raised every time (mainly from the early chapters of Genesis) to "prove" this point, completely ignoring the vast bulk of biblical literature that ranges from poetry and psalms, to the prophetic books, the stories of the patriarchs, the exodus, the periods of the judges and the monarchy, wisdom literature, court tales, second temple historical books, and apocalyptic.
To answer your question, I suggest that instead of being obsessed with sexual matters you focus on the real history behind the early chapters of Genesis - that of the Babylonian exile of the 6th century BCE. The two creation stories of Genesis 1-3, the flood story, the account of the city of Babel and the genealogies of this portion of Genesis all see to derive ultimately from Mesopotamian originals. The simplest explanation for this borrowing, which is largely unparalleled in the rest of the Tanakh, is that it occurred during the exile. This event provided both the opportunity and the motivation for the satirical rewriting of older myths that include Enuma Elish, the account of Enkidu and Shamhat in the Gilgamesh epic, the Sumerian king lists, the Mesopotamian flood story, and the tale of Enmerkar and the lord of Aratta.
What is the writer trying to say it was like in Noah's time? I believe he is trying to say that the "violence" of the antediluvian era was akin to the violence with which the Neo-Babylonian empire conquered Judah, slaughtered thousands of its inhabitants, destroyed its cities including Jerusalem, and took a huge number of slaves into exile in Babylonia in the 6th century BCE. Just as the Egyptian armies are defeated by drowning in the later story of Egyptian exodus, the flood story almost certainly functions as a parable of the eventual overthrow of Babylon by the Persians, allowing Jewish exiles to return to their own land. This underlying story is the one that is relevant to the world today, not a borrowed myth about animals and rainbows, a horrific slaughter that accomplished nothing, and an imaginary flood that never occurred.