Probably the most defensible ethical meta-view for an atheist is consequentialism as it seems to rest on an incontrovertible principle that harm or injury to self-interest is wrong. I don't agree that it's defensible, and the Nudger and I have debated this to no conclusion, but it has the virtue of being intuitively appealing and not vulnerable to the objections normally lodged against relativism.
I, for my own part, don't claim to know how morality works. Given that I can point to problems with theist morality theories, that might be a reflection of a lower vulnerability on the issue. But I don't claim to know where morality comes from, nor whether it's objective or subjective. I don't know how to reconcile relativity with quantum mechanics either, but I'm confident they can be reconciled because if they can't, then the world wouldn't exist as it does. Perhaps when someone challenges you on the how and why of morality, you can remind them that before Darwin, the accepted explanation for the diversity of life was God. That didn't mean that God was the cause of the diversity off life or that evolution didn't function before Darwin, it just means that we didn't have an answer before there was the theory of evolution. God was just an assumed explanation, and one that appears to have been wrong.
I, for my own part, don't claim to know how morality works. Given that I can point to problems with theist morality theories, that might be a reflection of a lower vulnerability on the issue. But I don't claim to know where morality comes from, nor whether it's objective or subjective. I don't know how to reconcile relativity with quantum mechanics either, but I'm confident they can be reconciled because if they can't, then the world wouldn't exist as it does. Perhaps when someone challenges you on the how and why of morality, you can remind them that before Darwin, the accepted explanation for the diversity of life was God. That didn't mean that God was the cause of the diversity off life or that evolution didn't function before Darwin, it just means that we didn't have an answer before there was the theory of evolution. God was just an assumed explanation, and one that appears to have been wrong.