The thing is that a bikini is supposed to be public and is made to cover the necessary in order to be as modest as possible while underwear is a private thing. Something intimate that only you SO sees and that is what you like. Your favourite lace, colour, style. It's a personal thing,
I don't really like walking around in bikini either, but like i changed my shirt in front of my friend who is a guy and it wasn't really very much difference, i mean we have swimdays in school together
ACTUALLY YEAH THAT'S A REALLY GOOD ANALOGY