How does the romance transcend the gender barrier here? To me it seems pretty heterosexual - for Mamiya i would think that with switching to female genetics, female brain, female body structure, female hormonal system and well, female everything, it would be reasonable to expect that change in sexual orientation would follow (but she's not the narrator here, so there's very little insight about how that happens internally). And as far as Miyoshi case goes, at the end of the day he's still in love with a woman (sure, his perspective shift about former friend turned crush turned lover, and conflicting emotions associated with it are pretty interesting, but they're not things that are handled in-depth by the author*). Leads wouldn't end up as a couple without the mcguffin fixed sex ratio magic (nothing wrong with that, just addressing the point).
I really liked it - it's sweet, it's heartwarming, it implies some nice things about attraction (while still staying true to it's evolutionary origin and being bounded by biology basis), but all things considered it's pretty ordinary shoujo (interesting premise and last minute sci-fi weirdness notwithstanding - though admittedly, that first thing is what sparked my curiosity and compelled me to start reading it). As a side note that i didn't know when to throw in, the art is really nice.
*in fact, as far as i remember, nothing is - everything seems just nibbled upon: societal ostracism of the emerged, familial abandonment issues, change in personal dynamics between people that have to come to terms with new forms of their friends, and probably other things that i'd forgot about already.