@Heimdal I was surprised by that - both of them /immediately/ went there with no hesitation. Thinking about the pair of them it seems a bit odd - Hino is still asking herself why she's got all these urges, and Koguma is just flat out oblivious, so jumping that far kind of suggests that they're both thinking a hell of a lot dirtier thoughts than we're seeing on screen.
Though thinking again it actually makes some sense, particularly for Koguma. After all, she did acquiesce (willingly, despite a whole lot of embarrassment) to the whole boob sucking thing, and her reaction to the idea of Hino in the sexy outfit wasn't embarrassment it was basically "my god that's hot" - there's definitely something going on behind the oblivious facade. If anything Hino is the one who's further from that, since she keeps approaching all this stuff from a perspective of teasing Koguma - she's more aware of it consciously, but she's a lot more separated from the whole thing, maybe because she's not the one on the receiving end of most of the stimulation . . .
Or maybe it's just a triumph of Japanese sex ed, and all Japanese high school girls in that situation would immediately spot the difference between sweat and "sweat" . . .