Actually, Jesus even reinforced the point about our fallen nature Himself in the Gospels when he corrected the Jewish people about divorce, speaking of the hardness of heart and later when He said "the spirit is willing but the flesh is weak"!

He's pointing to the lack of perfect dominion over ourselves, but "it was not so from the beginning."

That we have a fallen nature is factual. It's observable every day in every human relationship and activity.