You say what you do not mean by 'morality,' but not what you do mean.
If you mean that you have a verbal, propositional sort of normative ethical theory that you have 'developed mostly for fun and the violation of which has no perceivable impact on your emotional state,' then that does not mean that you are lacking in morality, it just means that your verbal normative theory is not in line with your wordless one. I do not believe that there is an arbitrary thing that you currently truly consider horrifying that you could stop experiencing as horrifying by the force of your will; or that there is an arbitrary horrible thing that you could prevent that would currently cause you to feel guilty for not preventing, and that you could not-prevent that horrible thing and stop experiencing the subsequent guilt by the force of your will. I do not believe that your utility function is open season.
I mean that what I call my 'morality' isn't intended to be a map of my utility function, imperfect or otherwise. Along the same lines, you're objecting that self-proclaimed moral nihilists have an inaccurate notion of their own utility function, when it's quite possible that they don't consider their 'moral nihilism' to be a statement about their utility function at all. I called myself a moral nihilist for quite a while without meaning anything like what you're talking about here. I knew that I had preferences, I knew (roughly) what those preferences were...
I have noticed that the term 'nihilism' has quite a few different connotations. I do not know that it is a coincidence. Reputedly, the most popular connotation, and in my opinion, the least well-defined, is existential nihilism, 'the philosophical theory that life has no intrinsic meaning or value.' I think that most LessWrong users would agree that there is no intrinsic meaning or value, but also that they would argue that there is a contingent meaning or value, and that the absence of such intrinsic meaning or value is no justification to be a generally insufferable person.
There is also the slightly similar but perhaps more well-defined moral nihilism; epistemological nihilism; and the not-unrelated fatalism.
Here, it goes without saying that each of these positions is wrong.