I’m going to be straight with yall here
(I'm a bad person, I know. I just couldn't resist that low-hanging fruit...)
I agree with you that the boy/girl dynamic being assumed is "normal" is a load of garbage. And it's lazy writing when an author treats that as the default. However! I don't think that's quite what's going on here, and it hasn't been from the very start.
I think Kazuki being "boyish" was actually because she thought it needed to be that way. Her previous girlfriends all dumped her because, basically, she wasn't a "good enough replacement" for a guy.
Obviously Hime starts out mistaking Kazuki for a guy, and falling for her as a guy. Even so, when Hime finds out Kazuki's a girl, she still ends up loving her with hardly any hesitation. As early as chapter two, Hime displays that she loves the feminine Kazuki just as much, and possibly even more.
Honestly, Hime has only shown immense love and pride for Kazuki as her girlfriend. If you look at chapters 2, 3, 4, 6, 7, and 9, they all have Hime being attracted to Kazuki the girl, instead of just the "cool" Kazuki. Also boobs.
In a way it could be argued that the whole premise of the manga is a repudiation of the notion that a relationship naturally has to have a masculine/feminine structure.
TL;DR, Kazuki's previous relationships followed the man/woman thing all the way through and didn't work. This relationship started that way by mistake and is now much better without it.