Let me try to simplify it.
I tell someone they cant climb over the fence, the other person ask why, i respond saying its trespassing. Now in your argument your saying you can, well obviously it can and does happen, in the same sense as slavery exist. My belief is that another person cant own a person(becauses its morally wrong), in the same way as some cant trespass, or cant rob a bank. When you watch a movie and a character tells another character you cant kill them, well yea this is, of course, untrue. Still its a moral belief, doesnt me it cant happen. Ill make sure i choose the right words around you next time though mr. sheep.