P1) If a belief is made true simply by mentally affirming that it is true, then it is not worth arguing about the belief.
P2) If subjective morality is true, then all moral beliefs are made true simply by mentally affirming that they are true.
P3) It is worth arguing about moral beliefs.
C) Subjective morality is false.
Defense of premises:
In defense of P1, it could be said that the truth-maker for the belief is immediately intuited by the individual subject. Since each individual will immediately intuit their own mental affirmations, there is no value in debating with someone else what one’s own mental affirmations really are. One need only introspect to discover the truth-maker for the belief, which is merely the affirmation of the belief.
In defense of P2, that is just what subjective moral relativism holds. If you define subjective relativism in some other way, that simply is not my target.
In defense of P3 I point to empirical evidence that moral debates and disputes are held to be among the most worth-while discussions humans have.
The conclusion follows from the premises, i.e. QED.
P1. If morality is relative to the subject, then morality is a domain that is a matter of personal opinion.
P2. All domains that are matters of personal opinion, are domains where facts and evidence cannot determine correct belief.
P3. All domains where facts and evidence cannot determine correct belief are domains that lack propositions for which it is worth dying before giving assent under coercion.
P4. Morality is a domain with propositions for which it is worth dying before giving assent under coercion.
C. Morality is not relative to the subject.
P1 and P2 are not too objectionable. That is just what we mean when we say that morality is subjective. I think that if you are going to object to the argument, you will object to P3 or P4.