Skimpy clothing - double standards - shyness or what?

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This is one of life's great mysteries to me. A lot of guys and girls seem really embarressed to be seen in their underwear yet think nothing of walking along a beach in really skimpy swimsuits etc. I have a friend who would never dream of wearing anything but baggy boxers as underwear but on the beach has the briefest pair of speedos I have ever seen! Whats the difference? Do they buy really naff underwear and are ashamed of it? Any views?