I presumed my parent was not primarily concerned with sustainability due to bits like dead orangutans and I like beef, but I sympathize for a fellow mammal. Sympathy and orangutans are not really part of the sustainability picture, which is a separate and large issue I don't intend to dive into right now.
Eating meat can be sustainable. I'm pretty confident if we all ate crickets, there would be practically no sustainability concerns, and we can't all eat quinoa as our protein source anyway. Chile is not able to grow enough quinoa to feed the world (quinoa is a picky plant)
The 100 / 110 is purely invented as a thought experiment, but I consider it to be representative. Your daily BMR is ~2000 Calories. Engaging in activity, sports, games, or mental exercises do increase your caloric consumption for the day, but your body adjusts quickly and your additional caloric consumption for the activity falls quickly into the hundreds of calories for all but the most strenuous activities. Plus, the more strenuous (calorie-expensive) the activity, the more likely you are to take rest days, which lowers your average daily caloric consumption.