I should clarify that the inequality I am referring to is the inequality of power - if no state exists and a mob of individuals agree to rob their neighbors, what will stop them? What is the 'gravity' that prevents evil, in dirtykarma's phraseology.
Our traditional methodology has been to vest a monopoly of the legitimate use of violence in a centralized actor whom everyone must defer to - the state. Perhaps there is another way to achieve this result, I am open to one, but 'natural law' (assuming it even exists) isn't prepackaged with a method of enforcement.
Equal individuals could agree to follow a 'natural law' for their protection and best interests, but if one individual saw himself as capable of subjugating everyone else, why wouldn't he?