I don't have any problem with the idea that a self-professed atheist or agnostic can tell right from wrong. A religious type will tell you that knowledge of "right" and "wrong" are written on our hearts, and that we therefore know the difference, even if we (fallen as we are) actually behave wrongly.
The question is, rather, do atheists and agnostics have any rational excuse for their opinions concerning right and wrong?
For example, if an atheist tells me something is "wrong," do I have to believe him? Note that the atheist must appeal to transcendant concepts to tell me that murder is wrong, as opposed to a merely sub-optimal choice. It seems to me that the fundamental precept of atheism precludes appeals to transcendent moral concepts.
As for an agnostic, when he tells me that something is "wrong", he's either going to have to justify his claims from the atheistic position (which won't work), or from a theistic position (about which he claims to know nothing).
There's no middle ground for the agnostic, since appeals to pragmatism don't work either -- pragmatism is an optimization approach, which is primarily based on answering the question, "what can I get away with?"
You mean that morality is just a matter of opinion and then, for you, it remains only to determine which argument is the most rational. My argument from natural rights seems the most rational. Rights from God seems less rational because I don't think there is such a thing.