Nah he's right. Something like a bikini is alright at a beach, but imagine seeing someone strip down while in a crowded train. It's all about the context.
Kind of unrelated but according to Baka-Updates, the author's female. She ( Tsurusaki Yuu) is also married to Ikezawa Shin, the artist of this manga.
https://www.mangaupdates.com/authors.html?id=16087
the author is a woman and is also probably right
if a woman appeared in front of me in what is basically but is not a bikini I would definitely assume she is promiscuous
Slut? Well, I wouldn't think her a slut.
I'd think her a weirdo.
Someone with a screw loose.
That she lost her marbles somewhere along the way.
An exhibitionist.
etc...