Seeing a lot of posts about this, but a lot of it just has to do with the context of seeing the girl. If you see her in a bikini, its most likely a public thing and she prepared for it and knew people would be seeing her in it before she even went out. If you see her in lingerie, its most likely private, which means she probably wasn't prepared for a stranger to be looking at her. (lot of underwear is also see through to some extent, which is another reason, but the above explains why even if the underwear is actually less revealing)
It's like say a person is fine with public speaking, as long as they have time to prepare and practice and everything, they'll be at ease doing it. However, out of the blue tell them they have to give a speech right then, and they'll probably freak out a bit. Even if the impromptu speech is shorter or on an easier subject (or even if the lingerie covers more than the bikini) because they weren't expecting it, they'll still freak out more about it.
Sometimes, that's not always the case.
It's like say a person is fine with public speaking, as long as they have time to prepare and practice and everything, they'll be at ease doing it. However, out of the blue tell them they have to give a speech right then, and they'll probably freak out a bit. Even if the impromptu speech is shorter or on an easier subject (or even if the lingerie covers more than the bikini) because they weren't expecting it, they'll still freak out more about it.