This is the face of a girl entering subspace.
Subspace is the altered psychological state of being that is often represented by actions, feelings, thoughts and images brought on by adrenaline and endorphin waves and spikes that may occur during a scene.
After the last few chapters I went back and reread everything with a D/s relationship in mind, and noticed that Koguma has probably been in subspace by the end of most of the chapters, after allowing Hino to dominate her. (Despite Koguma's resistance, she's not actually being forced/tricked into anything.)
In chapter 2 Hino kabedons Koguma and tries to get her to say exactly what she had thought she submitted to in chapter 1. She struggles to say it, and once Hino relents, thinking she might be going too far, Koguma grabs her shirt to stop her from leaving. Because Koguma wants to made say it. Being explicit about her lewd thoughts, because someone else made her, feels good. (Thoughts she probably feels ashamed for having.)
In chapter 6 after submitting to Hino touching her body, and basically begging for more, Koguma says:
"I feel so weird"
Here Koguma is probably having an intense physical reaction to assuming her submissive role. Sure she is turned on, but she's probably also unable to think clearly, tingling all over, face feeling hot, vision a bit hazy, etc. Having experienced subspace I can say that it could easily feel like you've been drugged with some sort of aphrodisiac. It can be an overwhelming experience.
In most of those pictures her eyes are glazed over and only half open. That's the face of a girl who loves feeling forced to do something that she actually really wants to do, (but probably can't admit to herself).
Prediction: Koguma will agree to being tied up/bound before the end of this series.