Our stomachs are built for very high acidity. Gastric acid has a pH value around 1, regular coke has a pH value around 2.3. To compare, lemon juice has a pH value of 2, orange juice has a pH value of 3. In other words, coke is not dangerous to the stomach. If anything, you should be worried about your teeth.

"Everything in moderation" is great advice for coke consumption. If you drink a gallon a day it will most likely affect your health. If you drink a gallon a month, you'll be fine.