I think tan lines give the impression that those parts are rarely seen or exposed. Or in reverse you get the opposite thoughts, if there are no tan lines you figure they must be ok with sun bathing nude (yeah fuck tanning salons ok). When many of us were reaching that stage in life where we were developing our sexual likes and dislikes in the appearances of women it was primarly sun bathing only, so it sort of stuck in our minds.
Sort of the less we perceive others get to see it the more we want to see it.
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