i think that this double standard stems from suggestion. everyone obviously has an opinion on how revealing swimwear should be... and that is their right. i personally am a wearer of bikinis and i don't find anything wrong with that.
i do however thing that it would be a little unnecessary to parade around in my bra and panties.
not because of the amount of skin exposed, but because of what is implied by those articles of clothing. swimwear implies that you are going swimming, while underwear implies that you are half way to naked. just like short shorts are less suggestive than an equally short skirt. the issue is not how much your showing but how you are showing it.
if you have an issue with bikinis, consider this... even a one piece bathing suit shows WAY more leg than any outfit to be worn in public ever should. that is without bringing up the issue of how form fitting all swimwear is. also, have we missed the fact that a man's swimwear is equal to a pair of boxers? Also they are made of different materials and designed for different purposes in mind.