It may be that women were being harrassed as sadly a woman who is dressed a bit tarty can be construed as a tart. Picking out women randomly at the bouncers discretion can cause a whole heap of problems so a blanket rule has been implemented.

Just a thought.... and add to that some of the attitudes here that if a woman is dressed a certain way in a certain place she may be asking for it.

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"Diplomacy is the art of saying 'Nice doggie' until you can find a rock - Will Rogers"