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.
Correct. That's how civilization works. A minority agitates for a moral cause; they convince enough people to have the law changed and society advances. For example, that's why we don't have slaves. In a free market slavery would still exist.
Its also how we reduced things like female genital mutilation and animal cruelty.
So yes - I am all for imposing morality.