Is slavery objectively wrong?
Does depend on what you define "slavery" to be, but in general terms....yes.
Slavery, even leaving aside the whips and whatnot, essentially means involuntary complete subjugation to the dictates of another. Doesn't matter how lovely that person is, it is the "involuntary" side of things that is wrong.
To be a slave means that you must do what your master tells you, even if
if horrifies or disgusts you or you feel it is wrong: you are a slave
, you have no choice. To remove the option of refusal, in slavery, is what makes it wrong, as it reduced the human being to the level of an animal - less than that, even, to the level of an object.