Sugar is fast food, by which I mean is converts into energy quickly. Eat a candy bar, and you get a "sugar high" that lasts for a little while. You pay for it, though. The high is followed by a crash, and much of what you don't burn immediately goes to fat. Too much of it on a regular dietary basis increases the chances you may become not only obese but diabetic, afflicted with cancer, weak of heart, and otherwise ill. Many believe refined sugar is more damaging to your health than raw sugars e.g. those found in honey, tree syrups and less cane juice. The industry insists there is no difference, that sugar is sugar.
There is no firm rule on how much sugar you can take in safely. Everyone is different in body mass, metabolic rate, level of physical exercise, and diet. The US Department of Agriculture guidelines, which are averages meant to promote maximum public (not individual) health, indicate that a maximum of 8% of your daily calories may be in the form of sugar. The American Heart Association restricts that more tightly, recommending lower ingestion to gain the best chance of avoiding health problems from it.
Another problem is that there is no way to know how much sugar you are eating. Most foods contain some. Processed foods tend to contain a lot more. It is in almost all the packaged products you can buy in the grocery stores. You can read the labels and calculate how much they indicate you are getting, but then you would be spending your whole day watching that, and no one has that much time.
For me, the rule is to cultivate a habit of avoiding foods that use much refined sugar, but I do not eat out much, and in my kitchen desserts are rare. I find I feel better and stay healthier that way, so out of personal experience I agree with those who argue that refined sugar is a poison. I think everyone has to do something like that to find his own tolerance level for it. The guidelines are not of much use, so I would recommend trying different intakes of sugar each over a period of perhaps a couple of months to discover what works best for you.