Saurabh, why do most humans perceive fair skin as sexually more attractive, is a difficult question to answer. Only the evolotionery scientists and experts in human behaviours would be able to answer that. There could be some evoloutinary reason for the humans to perceive fair skin, symmetrical face-shape, tall stature, hour-glass shape, melodoius voice etc as good. Humans, who possess these traits, generally fair better in the mating-game.
To me, personally, fair skin and dark hair appeal more than the dark skin and light-coloured hair.
And I have no guilt about my choice of likes and dislikes.
I will put it this way- It not a crime to like or dislike a particular physical trait (like skin colour) , but it would be unfair to mistreat or disrespect people who are different from what I like.
And when refuse to watch a film where the lead actress is dark-skinned and go watch the one which has my favourite fair-skinned lass, I am not disrespecting anyone, I am just respecting my own choice. She can act in any number of flms she wishes to, and without any make-up, it's none of my business; but she can't force me watch her films....)
It's as simple as that.