Isn't that you imposing your morality on other people? For example I am opposed to animal cruelty. Is it your argument that people who are cruel to animals that they own should not be prosecuted? What gives you the right to dictate that?
If a homophobe claims gays should not marry, but the gays say "We should be able to marry, to tell us we can't marry is immoral", are the gays now imposing their morality on the homophobe?
Anyway to answer your question: you begin by claiming the other person has no morality. You then assert your own morality as the only morality in existence. You can then easily dictate what is and is not moral.
