Close/open-mindedness isn't about being morally wrong/right. It's about holding a vile idea in your head long enough to see its various facets. To study it and understand it.

Openmindedness here means being able to understand that slavery is wrong at the same time you study what it reveals about the human condition. To be clear, no one in this thread has argued that slavery was good. If you think that, then maybe it's a sign that you're not separating subtle ideas well enough.