[Fish switching sexes] is absolutely not where you want to plant your flag if you’re trying to convince people — kids or not — that somehow transsexuality or the breaking of sex-based behavior (i.e., “gender roles”) is somehow natural. In the first place, what fish do sexually is completely irrelevant to what we do; it’s the same as an argument that we should all have unisex bathrooms because bacteria don’t have sex. It’s ludicrous.
Furthermore, trying to make these arguments in support of a tolerance agenda is a losing battle. If natural evolutionary sexual behavior is where you want to have your battle, those who argue against such things have nature pretty squarely on their side: the fact that other species have three sexes, or whatnot, means very little. Humans appear to have evolved to be sexually dimorphic, and with certain broad behaviors that signal masculinity and femininity. The fact that fish change their sex naturally just emphasizes the fact that humans don’t.