It's interesting that they explicitly show her inner sexual fantasies while still retaining that "pure of heart" vibe. I'm surprised that they manage to deliver such an illicit scene without disrupting the cute yearning they have going on. Because like, when you want to kiss somebody, when you're experiencing the start of your first love, of course those thoughts are swimming around in your head too.
I feel like so often in manga they characters will basically pretend nothing exists past the "step" they are at the threshold of before they cross it. That, or they'll have one character thinking about it and the vibe feels like "what a pervert they are to think about sex before they've hit X milestone."
This is very impressive, at least to me, because they've humanized both characters with these feelings, without implying some sense of degeneracy or impurity about it. They're still cute, they're still falling in love, there's still a romance. That's so important when frequently it feels like there's a big separation between love and lust in media, but especially in manga.
Not to fellate the mangaka too hard, I don't even think they were intending for it to hit that way. It did for me though, and that's what matters in this moment.