Well it is all about social norms, isn't it? We are accustomed to certain outlooks depending on the environment we are in. We sexualise undergarments because that's what the media does, bikinis aren't sexualised so we feel comfortable wearing them in public.
They may be the same thing but they will result in completely different reactions.
Thank you :)
Bikinis are less revealing. At least, usually. Because underwear often has a bit of see-through in them. Because everyone is doing it and it is acceptable. It is a suit meant for swimming and many people swim at the beach. You don't see people swimming in winter coats at the pools, either.