There is effectively no difference between this and this apart from the implied social context . The point is that there are fabric and construction differences between a bikini and bra and panties set . When you wear them , you can tell the difference . You are far more exposed in a bra and underwear than in a swimsuit even when they cover the exact same area on the body . The question focuses on the why wearing one in public dry is considered obscene and the other is not . Again , a swimsuit is made of thicker material and can has multiple layers and is constructed to be worn in public and stay opaque when wet ie it ' s thicker . It covers up areas that are usually considered private genitals and hence obscene if shown in public . Underwear is considerably thinner and shows a lot more details of what ' s underneath it than a bathing suit does , regardless of whether it ' s wet or dry . Personally , I think that it should be legal to walk around in underwear or butt naked and that Americans are far too sensitive about these things . But it is not true that there is effectively no difference between underwear and bras and swimsuits .