Is not your speech an extension of your biology? Thus it is ordered according to natural principles. This is the fundamental point Cicero makes. So while, yes, it is his "opinion", to deny a natural foundation to any morals is itself incoherent and not worthy of entertaining for dialogue, because dialogue itself presupposes the pursuit of the correct mode of living: wisdom.

And please make the distinction between "most humans" and exceptional humans. No doubt we all have a sense of right, but it is in its fulfillment - there's the rub!

I agree that morals arise from nature, in the same way that all things do, since there is nothing that does not arise from the physical. In that sense all things are natural.

I don't think that's what natural principles or natural law mean, and some brief wikipedia reading confirms my suspicion -- this is equivocation. For example, communism and authoritarianism also arise from speech and reason and ultimately human biology, so they are also natural in the sense you referenced. But Cicero would not have endorsed either of those forms of government and would not consider them part of his natural law.

Just because speech is biological that does not automatically cause it to be moral, only that it serves, as Plato describes, the "self-moving power" of Life. So then, communism and authoritarianism are contradictory to Nature, as has been visually demonstrated.

I highly suggest Reading Cicero's Laws, it's quite an original reflection and condenses Western classical thought brilliantly - it after all inspired the founders of the American Republic: http://www.nlnrac.org/classical/cicero/primary-source-docume...

P.S. If you believe "nothing [does not arise] from the physical" then how do you explain the ordering principles of logic? Are you to suggest that the mind does not exist independently of the physical brain? If so, where is your evidence?