Almost reminds me of Yuu from YagaKimi. Yuu, obviously, doesn't seem as dense to the prospect of relationships between two women as Maki does here, but they both clearly suffer from a lack of sense for the passion involved with a romantic relationship, as you pointed out.
Well, more like she's completely overwhelmed by the intensity of the emotions involved. That kind of thing can be pretty jarring and bewildering at first brush in general. To give a parallel one reason people tend to "freeze up" in dangerous situations is they don't know how to cope with the sudden rush of adrenaline - something appropriate training and experience helps a lot with.
You'd think this girl wouldn't have gone through with the whole thing if she didn't know what was going to happen, but I suppose Kousaka was being a little pushy.
I think it was more of a "I have no idea what's going on and it scares me but I'm also excited and curious and trust her" kind of thing on Maki's part. Such an anxious jumble isn't exactly unusual for teenagers having their first time I understand - nevermind now that the idea of two women doing it wasn't terribly familiar to her at the time, further adding to the disorientation.
a girl's first time is hardly ever anything like what you see in most medias. at least, what's going through their head when it happens. at first when i saw her say, so that was sex i was like gtfo of here, but then reading some comments got me thinking, and about my own first time. it wasn't that she didn't know about sex, but what happened didn't register to her as such at the time. and because of her age, with ZERO might i add experience (that we know of) of romance, she was completely and 100% not ready for that. even for girls who are, part of them still doesn't really understand that she's really doing it, and take time after to process it, just as jinno did. because for most, the act of having sex, is like crossing this boundary into a new adult world, cause there is very little else you can do that's any more adult like, at least for a kid. and a lot of kids especially at that age, don't REALLY know what actually counts as sex, cause like stated, most just see as penile penetration is all there is and rest is foreplay. She was not ready, there had been no romancing and leading her to that before hand (for more than the minute it took kousaka to get her in the room) and too trusting which made her too dazed when it started to happen to be able to stop it. she might later decide she liked it, but from what i got reading it, i think a part of her felt scared and maybe betrayed, which is why she was dazed/not really believing it was happening. felt very rapey to me, especially when it showed her covering her mouth with tears leaking out.