Why do Christian girls not expose their bodies?

Because the Bible teaches that Christian should not act unbecomingly! 
Christian girls dress decently without exposing their breasts and other body parts unnessarity. Christian girls are like this because they do not want to act in an unbecoming, inappropriate way. 
...DOES NOT ACT UNBECOMINGLYLY: it does not seek its own, is not provoked, does not take into account a wrong suffered.        
                         .      1 Corinthians 13:5

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