That was a very interesting, and for Japan, a surprisingly progressive idea to find in a silly manga about supernatural school mermaid love; that it recognizes the gender that a person identifies as. That's really neat and nice to see (even though like every tale this shit isn't gonna work out well for anyone involved). Also because I'm not a dumb sack of shit, this isn't exactly stretching the rules or laws of the mermaid stuff that they established. The person identifying as a woman, "having a woman's heart", is for all intents and purposes a woman so the rules for the mermaids aren't being changed or added to they're in fact remaining consistent.