@InsaneInsomniac Personally I'm too embarrassed to even wear bikinis, but I think it's about the setting. Anywhere you're wearing a bikini there's a bunch of people with tons of bare skin, pretty much anyone that'd see you is in the same boat which makes it less weird, and it's 'meant to show'.
There's something a lot more embarrassing about someone fully dressed seeing your panties when they're not meant to be seen... though even then I think the stereotypical girls who freak out too much are also going a bit overboard. If you're wearing a skirt it's probably an inevitability that you should be prepared for, I bet a few people have seen mine.
The fact that your underwear being seen from under a skirt is embarrassing pretty much shows it's not just about skin showing, 'cause that generally won't be skin showing.
As for if you're talking about like, getting caught without proper clothes on and just underwear... I mean yeah? Then you're being seen in a situation you absolutely weren't supposed to be seen and weren't expecting to be seen, by someone who isn't on the same level as you, and maybe shouldn't be where they are, lots of grounds to be unhappy.
Also a lot of fictional girls that freak out about that kind of thing and do shit like slapping someone who sees them aren't necessarily representative of most real girls I will say.
But yeah, there issss at least a little element of bikini vs underwear that is weird, I agree, but connotations matter. Society is what decided having bare breasts was embarrassing in the first place, really, so whether a bra is more embarrassing than a bikini comes with the territory.
Bit long winded of me my bad.