Most Helpful Guy
I have always wondered the same thing, especially considering that often underwear covers more than bikinis. But not all women are like that. I have been around many women who are comfortable being seen on heir underwear, at least by friends.
Most Helpful Girl
I think it's conditioning. We've all been conditioned to cover, hide, and cloth ourselves over our bra and underwear, and though you say the two articles of clothing are the same, underwear has a tendency to be a bit more lacy, or sheer, or look like underwear as opposed to the lycras and patterns and shapes of swimsuits, and besides, not all women hit the beach in a bikini because they do not like to show everything off, so it literally is not the exact same for them because they wouldn't be caught in a bikini on the beach anymore than walking around in their underwear.