Thanks nwysca & mrrple!
I use that line regularily--it almost always works. It's especially good when people are trying to use the "natural vs unnatural" arguments or "its ok because we're a higher being" or "hypothetical apocalyptical desert island scenario" reasoning. Saying something like
"you believe A and I believe B, and you might be right or I might be. But I KNOW that I don't have to hurt anything and can remain healthy and active, so I don't see why I should."
It really reminds them that it's not about natural or unnatural, and that they can say whatever they want about nutrition and pain and sentinence--in the end you're living proof that a normal healthy happy individual can live without hurting something, and no matter what they say won't change that.' Also, you've already said that they might be right--it's really hard to argue with someone when they've already accepted your argument. :P (You can, as I do, personally believe they're wrong--but is it worth arguing over?)
About coming up with lines: While there was a point where I had to come up with new responses, I also did my fair share of just getting upset. Feel free to take my lines (and anyone elses, I think there's a response thread) and make them your own--most of the replies I gave her were ones I'd said a million times over by now, lol--practice helps!