I can't imagine what they're referring to when they claim his books are sexist. I'm pretty sure as many protagonists are female as male, and a significant fraction of the characters have no gender at all.
Of those who do have gender, it's not at all unusual for them to have changed genders once or more in their lives, and the only time I remember anyone making a fuss of it is as part of Genar-Hofoen's character arc in Excession, wherein he is shown to have grown as a person by doing exactly that.
Can anyone point out an example of what the author might be thinking of? I'm genuinely baffled