Why do girls go in public with bikinis but not bras?

For some weird reason, it is more socially acceptable. Bras and panties are seen as "erotic" while bikinis are seen as well, swim wear (if you can call it that) even though it basically covers nothing. Girls can wear sports bras in public, but only because its athletic, not "sexy". It's kind of how guys can take their tops totally off in public, but showing your boxers is unacceptable.