Check out the videos of Dr. Lusting (the guy has over a million views on his youtube lecture about sugar).  The problem is the fructose (sugar) in the processed food combined with the fats.  Fruit in moderation is OK because it also contains fiber which slows down the absorption of sugar.  And you would have to consume a heck of a lot of watermelons for the sugar to be equivalent to what is easily accessible in processed crap.