Without a moral objective standard, we cannot say that slavery was ever, is ever, or ever will be, morally wrong for everyone. We can only say that it violates my personal moral standard. Or to me it's morally wrong. Or I think that is morally wrong.
Since militant unbelievers claim that no moral objective standard or universal moral standard, exists, then all morals or morality can only be subjective. That morality can only be judged good or evil from person to person.
That is not quite correct. We militant unbelievers sometimes conclude that moral subjectivity is a circunstantial, albeit persistent, fact.
You may want to read a bit about Utilitarianism and particularly Peter Singer's ethical proposals sometime. He is a very solid, very serious, very insightful author on the matter - and quite frankly, he's way ahead of most organized religions when it comes to this field.