You are correct in that conclusion. I think it is impossible for one to act on their own (true) moral preferences yet morally wrongly.
There are two remaining points, for me. First is that it's difficult to figure out one's own exact moral preferences. The second is that it becomes extremely important to never forget to qualify "morally wrongly" with a parent.
Frank can never act on Frank's true moral preferences and yet act Frank's-Evaluation-Of morally wrongly.
Bob can never act on Bob's true moral preferences and yet act Bob's-Evaluation-Of morally wrongly.
However, since it is not physically required in the laws of the universe that Frank's "Evaluation of Morally Wrong" function == Bob's "Evaluation of Morally Wrong" function, this can mean that:
Frank CAN act on Frank's true moral preferences and yet act Bob's-Evaluation-Of morally wrongly.
So to attempt to resolve the whole brain-wracking nightmare that ensues, it becomes important to see whether Bob and Frank have common parts in their evaluation of morality. It also becomes important to notice that it's highly likely that a fraction of Frank's evaluation of morality depends on the results of Bob's evaluation of morality, and vice-versa.
Thus, we can get cases where Frank's moral preferences will depend on the moral preferences of Bob, at least in part, which means if Frank is really acting according to what Frank's moral preferences really say about Frank not wanting to act completely against Bob's moral preferences, then Frank is usually also acting partially according to most of Bob's preferences.
It is counterintuitive, I'll grant that. I find it much less counterintuitive than Quantum Physics, though, and as the latter exemplifies it's not uncommon for human brains to not find reality intuitive. I don't mean this association connotatively; I don't really have other examples. My point is that human intuition is a poor tool to evaluate advanced notions like these.
This is sensible enough as a theory of morality, but you still haven't accounted for ethics, or the practice of engaging in interpersonal arguments about moral values. If Bob!morality is so clearly distinct from Frank!morality, why would Bob and Frank even want to engage in ethical reasoning and debate? Is it just a coincidence that we do, or is there some deeper explanation?
A possible explanation: we need to use ethical debate as a way of compromising and defusing potential conflicts. If Bob and Frank couldn't debate their values, they would probably have to resort to violence and coercion, which most folks would see as morally bad.
I think there’s a confusion in our discussions of deontology and consequentialism. I’m writing this post to try to clear up that confusion. First let me say that this post is not about any territorial facts. The issue here is how we use the philosophical terms of art ‘consequentialism’ and ‘deontology’.
The confusion is often stated thusly: “deontological theories are full of injunctions like ‘do not kill’, but they generally provide no (or no interesting) explanations for these injunctions.” There is of course an equivalently confused, though much less common, complaint about consequentialism.
This is confused because the term ‘deontology’ in philosophical jargon picks out a normative ethical theory, while the question ‘how do we know that it is wrong to kill?’ is not a normative but a meta-ethical question. Similarly, consequentialism contains in itself no explanation for why pleasure or utility are morally good, or why consequences should matter to morality at all. Nor does consequentialism/deontology make any claims about how we know moral facts (if there are any). That is also a meta-ethical question.
Some consequentialists and deontologists are also moral realists. Some are not. Some believe in divine commands, some are hedonists. Consequentialists and deontologists in practice always also subscribe to some meta-ethical theory which purports to explain the value of consequences or the source of injunctions. But consequentialism and deontology as such do not. In order to avoid strawmaning either the consequentialist or the deontologist, it’s important to either discuss the comprehensive views of particular ethicists, or to carefully leave aside meta-ethical issues.
This Stanford Encyclopedia of Philosophy article provides a helpful overview of the issues in the consequentialist-deontologist debate, and is careful to distinguish between ethical and meta-ethical concerns.
SEP article on Deontology