Actually, that's not really true. It's more about quantity than anything else. It is true that fast sugars invoke a greater insulin response, and that can be both good and bad. Small meals, often, even with sugar, are much better than large meals infrequently. You NEED some sugar to drive insulin response, anabolism, and to drive appetite, and, in a Real Jock, sugar post workout is BETTER for glycogen replenishment. Nature ABOUNDS in fast sugars in fruits everywhere. You'll what to study up on carbs to obtain a fuller understanding.
Understanding the relationship of insulin production, exercise, and lean muscle mass are important.
With regard to cows, their stomachs are designed for eating grass, and corn sugar (i.e. corn) put to them in the amounts to get them to market compromises the cow's health (they get fat), but, that's all relative: the cow is headed to the slaughter house in the fastest way possible so that folks have inexpensive protein.
For folks who want to optimize training results sugar can be essential in helping with anabolism and getting insulin to the point needed to optimize that anabolism. You'll have much better training results with simple carbs in your diet, especially post workout in the golden hour for carb uptake.
That understanding is why we now see lean bodybuilders at 300. They understand that relationship.
If you've seen "skinny fat" it's because they don't eat the right things in the right way, invoke insulin in the wrong way, and don't get adequate calories.
Americans are fat not because sugar is evil (it's in nature everywhere) but because they eat large portions, slam it with HUGE amounts of sugar in the form of soft drinks, and that sugar invokes an insulin response that store all that fat, and carbs, and they don't exercise. I eat 4800 calories a day to get lean, and I slam candy while I'm doing it, but, the trick is in being active, eating small meals, and not eating so much sugar (like soft drinks) that you end up with 100grams of simple carbs that tips your insulin up side down, which then causes you to go hypoglycemic which then causes you to binge.