The consensus does indeed come from somewhere: within functioning social groups. Without social groups there is no reason for morality. Collectively murder will not be acceptable one day because this would break-down functioning social groups; Homo sapiens and other apes live in tightly knit family groups and the total collapse of these gatherings would be fundamentally disastrous.
Let me turn my question into something a bit simpler: since people seem to believe that humans get their morality from god, and not from the learned application of functioning social groups, and that if this god says that rape is bad, murder is bad and perjury is bad then that's fine, god said it, you adhere to it; hypothetically speaking then, if god says that rape is good, by your reasoning you should now deem rape as a moral behavior! Unfortunately this is exactly the method religious zealots use to legitimize atrocities from the killing of infidels to the banning of condoms in Africa. The reason we can condemn such scenarios is because god does not dictate morality, it is integral to ourselves and in fact our morals have evolved alongside us as our culture changes.
As a further example that shows the fallacy of this reasoning, in the bible, kin selection is very strong. The so called "ten commandments" specifically say that one is not to murder; this dictate is supposedly from god. Yet within a few mere pages of these “newly-revealed” laws the Hebrews are out slaughtering whole cities and raping virgins, supposedly on the order from their god. How does one reconcile this ugly contradiction? It seems to an outside viewer that the laws were meant only for the direct kin of the Hebrews and not for everybody, hence the guilt-free massacre of thousands. The Hebrews were exhibiting a classic kin selection and family group mentality and had no idea of individual human rights or the morality you and I take for granted today. I submit that this story is a perfect example of what evolved in us from natural causes. Today's morality comes from an adult and civilized struggle to figure out how to live with one another, and by and large the results have been fantastic!