Listen bud, If you prefer your sis,wife or mom to be able to dress in a bikini or a modest dupatta or whatever she wants.. good for you.
Now comparatively, if you feel like walking around in your skibbies that too is your choice, please do so.. since it is equality we are looking at here. But if a person feels uncomfortable with glaring eyes at her and decided to wear something to cover any part that invites such a response.. its her choice. Now as to why Islam promotes a certain degree of modesty...just answer this. (graphic description follows.. discretion is advised
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You are in your office and a woman wearing a tight dress with a low neckline bends down in front of you...her well curled hair drops in front making her look like a godess and you see all that most men desire to see and will keep your eyes glues on till the last possible instant.
If you are a normal straight male your Bp will register a measured rise, and a hormone release will follow which will trigger all sorts of bodily events and your imagination will run wild for the next 15 to 30 minutes and every other time you see her. This woman however is from an otherwise nice family..and would not dream of those acts that are cooking in your mind right now.
Now if the same woman wore a proper modest dress with a headscarf.. unless you are a thorough pervert.. you will pass a casual look and return to your work, your mind did not mentally assault her and her respect in your eyes will remain.
Now if in your place there was a man with a high probability to assault a woman to get what he wants.. who is safer?.. the woman who chose to dress a little provocatively or the one who chose the covering?