If morality is not objective, than moral propositions do not have true-or-falseness about them, and all the discussions about morality are vapid.

It seems very obvious and uncontroversial to me that morality is not objective. (Yay typical mind fallacy!) Morality is, or arises from, a description of human actions, judgements and thoughts. Aliens who behaved completely differently should be said to have different morals.

It's not clear to me why someone would even think to argue for objective (=universally correct and unique) morals unless motivated by religion or tradition.

Of course, it's also clear to me that our subjective morals add up to normality. For instance murder is generally morally wrong. Of course that should be read to say it's wrong in our eyes! Of course things are not right or wrong in themselves; value judgements, including moral values, are passed by observers who have values/preferences/moral theories.

It seems to me that nihilism, if it is commonly understood to mean this, should be accepted by pretty much any materialist. This doesn't seem to be the case. What am I missing? What are the reasons to think there's something in nature (or in logic, perhaps) that should be identified as "objective morals"?