There would be no business ethics, no animal ethics, no engineering ethics, no education ethics, maybe political ethics-but probably not, in other words almost all fields of practical ethics would cease to exist. These fields would largely not exist or be changed so as to be unrecognizeable under your suggested definition of ethics because very little of any of these fields of applied ethics has anything to do with law. Again this is a simple statement of fact compared to your criteria of ethics. We need a much better word, or a set of words, but then we would have to agree on what morality is and that's the big public struggle. The general definition is too vague to be meaningfull without some elaboration.
Sex is never immoral.
Force or fraud, that is immoral. On the contrary, I find this makes the meaningful discussion of morality possible. First is that 'good' by which I assume you mean morally right, is presupposed, which is of course circular reasoning. Second fallacy is that in your claim that discussion could only be between inherently simillar beliefs, you presuppose that morality is in fact subjective, again relying upon circular reasoning. What is subjectively beneficial is not what is moral.