Koguma is embarrassed, but she is clearly into it. She can get away whenever she wants, because Hino has never actually forced her into anything. Aside from locking the door (not a big deal when it's locked from the inside and Koguma can easily open it herself) she's not even done anything unusual this time around. Her excuses/tricking Koguma into doing things for her is nothing new.
This doesn't require a "safe word" or any nonsense like that, because this isn't some play. It`s just teasing. Hino compliments her because those are her true feelings. What girl would not get excited when her crush says she is cute and says her body is her type?
The genius of Hino is that a lot of her moves cut both ways--the sudden-locking-of-the-door trope always signals secrecy/privacy, so sinister overtones are never far from the surface, but Hino's explanation that she doesn't want Koguma to be embarrassed if someone else suddenly came in is perfectly plausible.
It's just that the visuals really hammer home Koguma's vulnerability--her postures and facial expressions often echo hentai victim poses but the drawings scrupulously avoid anything like lewd physical details.
https://dynasty-scans.com/chapters/hino_san_no_baka_ch17#10
So it sorta feels like Koguma is being victimized (or about to be) when everything else in the story shows that she's really not.
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