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The arguement that wearing the burkha dor not harm women is not enough. Should people be able to choose slavery? There are good arguments for this by Muslims in Muslim countries. But, our taboos say no, even if hat means complex laws that define the limits of contracts.
Willingness to be abused is not sufficient arguement for the burkha. Even where prostitution, for example, is legal, sex slavery is abhorrent and against our laws. Society does have its right to norms.