Zachary: One would have to be blind to the reality of moral obligation to deny that people who believe a course of action to be morally obligatory for them have both the right and the duty to pursue that action, and that no one else has any right to stop them from doing so.
Zach's Principle
If you believe action X is morally obligatory for you, then you have the right and the duty to pursue action X, and no one has the right to stop you from pursuing action X.
Cynthia's Objection
What if person A feels a moral obligation to prevents artworks from being destroyed, while person B feels a moral obligation to destroy pornographic artworks?
Evaluate
Cynthia's objection is saying that,
So the art-preserver would have the right to preserve pornographic art, which would infringe on the art-destroyer's right to destroy that pornographic art.
Goal
We need another example in which one person feels a moral obligation to do X, and another person feels a moral obligation to do not-X.
That preserves the logical force of the objection.