H O N E Y

Why are girls embarrassed when in a bra and underwear yet eager to be in an even more revealing bikini?

Consent.

If a girl is out in her bikini, she wants to be seen. She is outside because she wants to be and she is showing her body because she wants to.
If you catch a girl in her bra and underwear and she’s embarrassed, then she did not consent to be seen. You have violated her personal space at a time that she does not want to be seen.

In one scenario she’s chosen to show her skin and in the other she didn’t.

It’s not that hard to understand.