@mikejacobs
Some slaveowners are bad therefore slavery is bad? By the same reasoning, some parents do extremely horrible things to their children, so raising children should also be outlawed. Free the children from their parents so they can't be abused!
Not saying your conclusion is wrong, but any time you give people power over other people, there will be numerous examples of abuse of said power. It's still necessary to have people in those positions of power in many cases, but if you have the right checks and balances you can prevent or at least punish the worst abuses. This would apply to slavery as well. There are strong arguments against slavery as a system, but "it's bad because bad people do bad things with it" is not one of them.
What makes slavery bad in most JP stories is what
@xyzzy said. If a main character bought a slave for whatever purpose, then gradually grew to trust her as he gained her trust, culminating in him freeing her from slavery so they could have a relationship on equal terms, I think that would be OK in terms of a romantic subplot. Instead, what typically happens is he buys her because she's cute, she falls in love because he doesn't abuse her immediately, and when he tries to free her she refuses because she wants to be owned by him.