The assumption of the question is that we can just intuitively "know" whether or not something suffers. My "queasiness" about doing something is determined by a complex array of sociohistorical, psychological, and biological processes (did I leave anything out?). For instance, I'm grossed out by the idea of french kissing my first cousin who's about my age. That's not because there is some natural or intuitive aversion to kissing first cousins (indeed, in some cultures, you're expected to marry a first cousin!); rather, incest is defined in my culture as including first cousins, so that makes me queasy.
[WS:] I agree with the main drift of your argument that there is nothing natural about the contents of ethical principles, they are all bases in a specific social, cultural and historical context. I would go further by claiming that any attempt to deduce universal or "unshakable" ethical norms from abstract principles or natural phenomena is pretty much misguided.
There is nothing unnatural or contradictory in extending or denying rights to anyone or anything. There is no logical reason why we should extend rights to minority groups, animals or even vegetables, but there is no logical reason to deny rights to any of those groups either. The only way to extend or deny such rights is by reaching shared understanding, a social consensus if you will, whether certain rights are alienable or not to for whom or what. Inasmuch as that shared understanding of core values is in place, people know what is right and what is wrong and feel a duty to follow it (albeit they may choose not to do what their duty is.)
I do, however, believe that universal ethics is attainable, but not through logical reasoning or utilitarian calculus, but through the simple concept of duty with which most people are born (just as they are born with the capacity to learn a language.) The emergence of such universal ethics will be accomplished in the same way as a common language eventually will - though social integration. At one point, all people will form one community that will speak the same language and adhere to the same core ethical principles that people will have a duty to follow.
This is pretty much a continuation of the same process that took place throughout human history: smaller social groups being integrated (either by force or voluntarily) into larger tribes, states, federations, and so on, and as a consequence developing shared language, shared norms, and shared identities. And this process is still progressing as the integration of Europe demonstrates. It is only through that process of integration that ethical principles become universalized i.e. accepted as valid by larger and larger numbers of people.
I may also add that from this point of view, deontologism (ethics based on the concept of duty) is perhaps the only hope to achieve a universal ethics. It stipulates that while a shared set of norms and principles emerge through social integration - those principles will be held valid for the entire humankind, and thus be truly universal (it does not mean that people will always follow them but that is an altogether different issue.).
Utilitarian ethics, otoh, makes such universal ethical less and less likely. As the society becomes more and more complex, calculating the Pareto optimum - i.e. optimum good for all people - becomes impossible (I think it is impossible to calculate it even today.) And since we have no way of knowing which actions add to common or "universal" good and which do not - the only logical conclusion is pomo nihilism - everything is justified, everything goes, and let the market sort everything out in the long run. This is the ethics or rather anti-ethics of capitalism.
PS. I am pretty sure that the universal ethic that will emerge from social integration of all humankind will extend equal rights to all humans. This is implicit in the concept of integration. If some groups are excluded, it means that integration is not complete, not yet. What I am less certain is whether rights will also be extended toward non-human species. I sincerely hope they will, and the ay things seem to be going today, this is not an unreasonable expectation. But again, all ethical norms are grounded in shared social norms and expectations, so it is possible that a different consensus will eventually emerge.
Wojtek