No other can determine this for us, for we each have different hierarchies of value. I do not value chocolate at allas a good thing so to consume chocolate is not a good thing in my life. I know for a fact that others believe differently about their own lives. Such decisions are about what is good for each of us in our own lives, but they are not identical with what is moral. It may be good for me to murder my poor old aunt Hildy who is wealthy and take her money since I know I can get away with it, but it is not right to do so.
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.