You've forgotten that part due to imbibing entirely too much wine and sleeping with those creatures called "the daughters of man".
Oh, yes, how far have the mighty fallen!
(NOTE: My theory is that if you accept one little piece of Genesis as a perfect reflection of history you have to make up stuff to fit it all in, else none of it is no good and you might as well become a Democrat, become homeless street bump, drink Ripple from mason jars, and sleep in cardboard boxes).