Ivankov changes himself into a woman when he feels like it, which happened once or twice throughout hte series, and also changes other okama when they want to, but they're explicitly attracted to men, which is hammered down in Sanji's timeskip experience

not to mention his power is to alter people's hormonal systems, which can do quite more than changing a person's sex, for example it can cure the most lethal venom (though at the cost of the person's remaining lifespan)

so yes, i've been reading the series for over a decade, thanks for asking