>We can’t have moral duties to do the impossible or control our involuntary functions.

Would that it were so. But this perspective trivializes morality, reducing all moral duties to a preference rather than an obligation. Self-determination is only a strong foundation for a theory of an ethical system if "ethics" is merely a quid pro quo voluntarily entered into (conscientiously and knowledgeably understood) by all participants, the very opposite of morality, though admittedly as close to it as a formal, conventional, or historic philosophical theory has gotten. But if historical systems of ethics had successfully deduced the nature of morality, philosophers would not still be discussing such things. "Normative ethics" is as inadequate for explaining what morality is, let alone elucidating its ramifications on how we should behave, as scriptural faith is.