What's the moral problem with pornography? As far as I can understand it, it

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What's the moral problem with pornography? As far as I can understand it, it hinges on the concept of 'objectification', which seems to mean treating someone else as a means to your own ends rather than as an end in themselves. But if I go to the corner shop to buy a pint of milk, aren't I treating the guy behind the counter as a means to my own ends (buying a pint of milk) rather than as an end in himself? Does buying milk have the same ethical status as pornography?