are vegetarians more ethical by not eating meat?

[littleendian]

I'm wondering why, rationally, ethics starts there, and not before?

Because we know that any one of us will do whatever is necessary to ensure his/her life, because that's just what life is at the minimum level, self-preservation and procreation. If any one of us in that particular situation would act no differently, then I think it is quite useless to talk about moral/immoral because it won't matter for our actions anyway. Morals exist to guide our actions, if the particular situation is such that nobody will ever be guided by morals anyways, then that I would argue is outside the moral realm.

There may be other problems with this, but it seems that you are saying it is okay to be immoral if my being immoral preserves something which I value (at some arbitrary level).

No, not at some arbitrary level, at the most fundamental level of life, namely self-preservation. There is a significant moral difference between killing a lion that runs at you or killing a rabbit when almost starving and accepting the death of a harmless cow so a well-fed person can have a steak.

why does a certain choice or act have no moral significance if my life is at stake, but the same choice have moral significance if it is not?

Because our higher faculties of reason and morals can only guide our actions when the lower levels of biology (survival etc.) are not overriding them, because at the core we are still the product of evolution, and at that level we are basically machines that first self-preserve and then pro-create. At some point the brain-stem just takes over and the cortex sits back and just watches with awe at how low the latency between appraisal and action can be in specialized hardware.

why does pain have any moral significance at all, rationally?

Because I don't want to feel pain, and that is rational, because pain is the body's signal to consciousness that there is something going on that is not in my personal interest, namely my physical integrity is in danger. We can argue about whether my will to live, which is expressed through pain, has any ground in reason, and my gut-feeling would say "no", but no matter what the answer turns out to be, it will apply equally to all animals, human or not. We all have as much or as little a reason to want to be here as any other. If I desire not to feel pain, how can I have any morals if I not first allow others the same privilege?

But ask yourself if it is not just your wish to keep the comfortable status quo that makes you and everyone else here so eager to see a difference in humans, is that rational?

Regardless of what my views are and whether or not I hold them rationally, neither has any bearing on the validity and rationality of your ethics. Please keep your red herrings and implied ad hominems to a minimum.

Believe me, I've endured my share of ad hominems in this thread, and they were quite a bit more insulting than this rather innocent question I've directed towards you. I'm certainly not engaged in this discussion because I'm collecting Kudos. But what red herring?

It won't convince you, and that's good, but I'll still appeal to expert authority from my favourite philosopher:

All beings fear suffering, all tremble before death.