Objective vs subjective morality

Does objective right exist? I remember debating my professor that there is no such thing, because we can never divest ourselves of our human mind. Everything that we conceive of is framed in human terms, relevant to our own experiences. For example, is it wrong to eat one's father? Our society, and that of the ancient Greeks recoiled in revulsion at such a notion, but Aristotle informs his audience that there was a tribe who ritually ate the bodies of their dead fathers, and concluded that for them it was right to do so. Thus we see that it can be both wrong (according to our norms and that of the Athenian Greeks) and at the same time right (according to the mores of Aristotle's quoted tribe). It all depends on our frame of reference. So where does this leave society and moral philosophy? If we cannot guarantee moral objectivity how can we make laws that can properly govern mankind? The answer is: act as if there are objective moral rights in order to halt the descent into brutishness and morally subjective chaos. Say that IT IS always wrong to kill. Full stop. Always wrong to injure, maim, torture; always wrong to oppress the weak; always wrong to defraud, lie, cheat, commit adultery. Always wrong to eat one's father. As humans we are too prone to slide about on the comforting grounds of acceptable attendant circumstance - he was in so much pain; the torture helped us find out where the terrorists were; they can't govern themselves; the banks have too much power and money; my wife doesn't understand me.....
Act as if there are moral certainties, although we can always justify the exceptions to our weak human selves, because that is the only way that society can function properly.