To Atheists: the Problem with Moral Relativism

Most Atheists concede that, without god, society is fully capable of determining its on set of acceptable ethics and practices. As we all know and realize, societies vary geographically. We can observe this through Eastern v. Western differences such as, but not limited to: religion, medicine and food. Therefore, a regional society finds it permissible to practice the act of cannibalism, pedophilia and murder. If there is no god to tell us that we're in the wrong, how can we condemn these acts? All 3 aforementioned practices can be rationalized as beneficial in some way - yet, I find it hard to believe any of us would readily join a society that allows for this type of behavior.