You make it sound like your decadences cost little ("10 percent externality") and give you lots of happiness, when it's the other way around -- things like eating cattle, fish -- things like driving -- have huge environmental costs and probably don't make you that much happier or better off.
1) I'm not talking about decadence, I'm talking about burning a little extra calories to engage in sports and games and such. Or choosing foods that are not the very best in terms of externalities, but still OK and also good for your health. You could live on nothing but rice, but it would totally ruin your health, and then have you not wasted all that rice?
2) Engaging in sports do not increase your average daily caloric consumption much. Once your body normalizes to your activity level, your BMR is something like 75%-90% of your total calorie consumption for the day. So the incremental cost is indeed low.
3) Fish can be a very economical meat. Feed conversion ratio is fantastic. To my knowledge most environmental damage comes from irresponsible fishers/farmers, not from inherent problems with fish. Crickets (one of the few even more efficient meats) are simply not available.
