"So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate." (Genesis 3:6 ESV)Which is an example of crossing a divide that shouldn't have been crossed, of spiritual adultery.
"the sons of God saw that the daughters of man were good. And they took as their wives any they chose." (Genesis 6:2)Same thing happening? Leans towards supporting the idea that this isn't human marriage (which is positively encouraged) but perhaps an example of angelic sin - with the "sons of God" being an angelic title (Job 1). When the LORD sees the aloneness of man in Genesis 2 he provides what's good for him - he doesn't have to take. Sin is usually the taking of something we see to be good - which probably is good, just perhaps not good for us. Just because something is good doesn't mean we're allowed to take it. Sometimes we're to wait and receive what's good. Elsewhere the LORD sees that things are good and blesses, and a wife will be found for Isaac who is seen to be good, and is then given to him.
In Genesis 6, the divine response to people crossing the divide is to remove the divides between sky and earth, land and sea, to answer human destruction with de-creation, taking the earth back to a formlessness (Genesis 8) awaiting a fresh wave of spreading goodness from the rest-bringer (Noah) and his seed.