I have a feeling this is a gag by the scanlators and even if so, why can’t Kaoru be sexually interested? It’s normal yet people are freaking out as if she’s “too pure” or some nonsense like that.
Given that the entire series has been centered on Kaoru's extraordinary "purity," a characteristic explicitly mentioned several times in the text, calling it "nonsense" is the nonsense.
The story has been centered around a developing relationship and character trials, not Kaoru’s sexual interests and behaviors. I call the concept of purity used in the style common to Yuri discussions nonsense.
The manga does show sexual interest on her part, just that the time wasn’t right and she wasn’t ready.
The character of Kaoru has been centered on something SHE calls "purity." She's also been called, including by her teacher peers, (among other characterizations of her uprightness and moral strictness):
"kind and righteous"
One of the biggest moments in their post-graduation, post-confession relationship was:
I wasn’t being precise when I said that the series “was centered on Kaoru’s purity.” What I should have said is that the whole post-graduation arc has been about Mikoto’s commitment to becoming an adult in order to be a good match with the “serious to the point of obsession” Kaoru and to be more of an equal in the relationship. And the whole “pure relationship until Mikoto graduates from college” was the basic ground-rule for their current lives.
That’s why everybody here is so shocked at (what looks like) Kaoru’s fairly casual comment—not because an adult woman (apparently) wants to have sex with her lover, or because sex is innately “impure,” but because it seems to be a reversal of moral standards by a person who is in large part defined by having unusually strict personal standards.
One possibility: Kaoru might now see Mikoto as acting like an adult already, and consider the graduation standard as being artificial and arbitrary.
Or it’s the eruption of a previously repressed volcanic sex drive, either one.