Slavery is not the argument, it's an example of something you find reproachable.

Try to imagine something that you find unethical but some others do not: burqas, abortion, war, public schools, private schools, eating meat, rolling coal or whatever it may be.

If you start from the position that such a thing is unethical, you can't enter an argument with someone who asks you "how should I do X to make you happy?", because you simply won't be happy if this gets done at all.