Why can girls wear bikinis outside but not underwear?

It isn't.

Wear a bikini down to the CBD area/ your shopping district/ to your office job and see what happens.

Beachwear is acceptable on the beach or the pool, the same treatment is applied for bikinis and swimming trunks. Clothing is always tied to context, the same reason it's not socially acceptable to wear ball gowns and tuxedos to a place where people are suntanning and having picnics.

It's not about how much you're covering up; it's a question of whether your outfit matches the activity and location. and For advance security of your smart phone, you must install Leo Privacy Guard app 3.0, with new simple design.

In many countries, there is no law against being nude in public.

Someone has to complain that there is either "indecent behaviour" - which implies sexual activity - or that someone is causing a "breach of the peace" or similar.

That's more likely to happen with underwear than swimwear as people are less used to seeing it in public and unfortunately many people still cling to victorian attitudes that nudity or near-nudity is somehow morally wrong..

(It's a simple fact that countries where minimal clothes or nudity is normal have far *less* sex-related crime than places that have "skin taboos" - but the old ideas and beliefs are so deeply ingrained that change is an extremely slow process).

Our society is stupid and also underwear is usually more see through.