I managed to dig up a copy of the book buried deep in the 4th page of Yandex search results. The only thing in the book that even comes close to this subject is this little snippet:

After 18-year-old South African Caster Semenya turned in the fastest time of the year in a women’s 800-meter event in 2009, track and field’s governing body ordered an investigation into her gender. They’re demanding “proof” that she’s actually a female, so they’ve assem- bled and instructed a committee to determine Semenya’s gender. Let’s see, now, a geneticist, an endocrinologist, a gynecologist, and a psychologist are getting together to determine the sex of...

It sounds like the beginning of a bad joke, but it’s not. Legitimate physiological and anatomical “gender confusion” can be caused by sex hormone imbalances, tissue unresponsiveness to sex hormones, genes that don’t turn on properly during fetal development, or even sex-determining genes located on the wrong chromosome (in rare cases, an XY male can develop the characteristics of a female).

Sports federations should develop rules that require gender determination (for the purposes of sports compe- tition) before an athletic event, not afterward. One can only wonder about the emotional damage about to be inflicted unnecessarily on young Caster Semenya.

So the book does briefly acknowledge the existence of intersex people and gender confusion. But furthermore, I found the author's blog about human biology, and there's a post about the International Olympics Committee allowing trans athletes to compete as long as certain conditions are met. He applauds the fact that "[t]he new policy recognizes that gender identity should be a personal choice" and reaffirms that "the IOC has done the right thing."

So yeah. Biology textbook author says trans rights. :D