@3ds1234 If you're a guy who's grown up with sisters you will have a different perspective on women compared to guys who don't have sisters (or only have brothers). The same is also often the case with women who have brothers. The ML I don't think has been suggested to have sisters so seeing underwear like that might still come as a shock not to mention there is a difference in seeing underwear of a girl you know vs seeing the underwear of
a girl you like. Granted he's seen her in her underwear (and less) but since they aren't in a relationship and culturally Japan has rather strict rules on personal space, he definitely will feel like he's encroaching on it (and if he doesn't he's probably an asshole.)
Remember, if something seems weird, think about how much of it comes from your own life experiences vs what others might know.