I think we need to go back to basics and skip the multiple point 200 word posts.
Tell me why you think stealng is immoral.
I base objective morality on moral realism (morals have a realness and truth about them outside human minds) and moral experience in real lived situations.
So stealing can be shown to be objectively wrong in a real lived situation not by what people say or claim morality is but by how they act and react in moral situations which cannot be hidden and often exposes what people really believe about morality even to the point of contradicting their own subjective moral views.
Most telling is how people react in the moral situation itself. So someone that claims the subjective moral view that stealing maybe OK for some like poor people when a poor person steals from them they react like its objective wrong to steal.
We intuitively know certain things are morally bad regardless of subjective moral views. We don't walk on by someone stealing a womens handbag and think "Oh well thats OK because the mugger is just acting out his moral views that stealing is OK". Rather we think someone has to stop the mugger and that is wrong.
If you look at society this way you will begin to see how even the most subjective moral claims by people are contradicted by the way they protest and condemn behaviour as being morally wrong. This is especially true on social media and even forums like this. People shame others, virtue signal, condemn others.
Its not just expressing views either but actually putting some truth out into the world about morality. It logically stands that an opinion or preference carries no punch because its like condemning someone for liking choclate. It just doesnt work.
So they want to be able to make a good arguement and truth claim beyond their subjective views otherwise its just a meaningless exercise and people don't really believe that their moral values are meaningless in the moral world they live.
If you look at most philosophical articles the experts agree that when people express moral views/claims they are actaully making some moral truth claim beyond them and into the world. There is also an arguemnet for why we should trust our moral experience as real and true but thats another explanation.