Here is an interesting essay by notorious bioethics professor Peter Singer. What is interesting is even Singer has realized over time that either morality has an objective basis or one is logically led to skepticism regarding any ethical assertion, even one’s own. This becomes somewhat ridiculous. We are stuck with philosophers who have destroyed any rational basis for ethical assertions, but who are constantly asserting what they believe to be ethical!
“If I deny that normative claims can be true or false, then I cannot assert that this claim is true. It too could be treated as just one preference among others – except that now there is no basis for saying that we ought to maximize the satisfaction of preferences.”
“The denial of objective truth in ethics thus leads not, as I had tried to argue, to preference utilitarianism as a kind of metaphysically unproblematic default position, but to skepticism about the possibility of reaching any meaningful conclusions at all about what we ought to do.”
Of course what he does with this change of mind is to still seek a purely materialistic ground and cause for an objective morality and I think he fails, but he certainly gets points for facing up to the intrinsic problems with believing morality can simply be reduced to subjective preferences.