The shenanigans involving Futaba's blood during the Tournament arc and immediately following suggest it was not entirely out of left field. According to later interviews, part of the problem was that the manga was canceled before the writer/artist was ready to finish it.
Fridge Horror: The ending, which reveals that, thanks in part to Futana's indifference to birth control, the Shimeru bloodline is spreading across the world. Implying that, eventually, humanity may end up being replaced by the Shimeru species entirely, since the defining aspect of the species, arousal-based gender-switching, is true-breeding.
A lot of Futana's actions. A Depraved Bisexual who leaves a string of bastards in her/his wake, she's such a pervert that she even hits on her own sibling, to the point Futaba can cause Futana to switch gender. To say nothing of the fact she's a disgusting slob of a party animal who, at one point, kisses her brother... so she can throw up in his mouth. Because he was closer than the toilet.
Really, the whole premise, which is pretty bad even if you make the assumption that, since Shimeru begin transforming as puberty, they're a inherently bisexual species.