I agree that they and similarly cut and cover the same areas; however, I personally can not think of a bikini as underware. Every time I see a girl in a bikini I just think of the beach and swimming. There is something about an actual pair of bra and panties that is far more intimate and personal.
I don't think it matters. It's fabric, but it still shows a lot! I think that a lot of women WANT to show off, and WANT to be SEXY!! I think it's in our genes for women to attract men, and for men to want to get with them to keep us going. . . Why is there so much anxiety, angst, and disapproval of a NORMAL, NATURAL way for the genders to get together? It's OK for women to wear bikinis, essentially their 'underwear' in public, and it is OK! THAT'S GREAT!!!