Noah's global flood never happened. It's a morality tale based on a myth borrowed from Sumer.You read it exactly the opposite of God's true intention.
God was done with almost the entire human race and Noach spent 120 years building the Ark and no one got the message that they were acting like predators.
You also didn't read the verses before God speaks to Noach...no surprise as you probably feel your membership in the Atheist Club will expire if you actually read anything in context.
The verse says that even the animals corrupted their ways; such is the power of perverse human behavior.
It could be the people were forcing inter-breeding in a manner antithetical to the laws set down by Adam.
Sounds like today, doesn't it?