[choeger]

> There is no amount of beef that is good for the environment.

That's a tautology. In that strict sense, there's no amount of you that's good for the environment, either. Unless we all commit suicide, we have to accept the fact that our behavior changes the environment. So, yes, quantities matter a lot.

[mklarmann]

If you define “sustainable” in the sense of “enabling human life on earth”, you should be fine.