You mean, other than the extra head?
(Not sure that's the best example to use.)
I'd also suggest
The Left Hand of Darkness it's held up as a seminal work on this.
BTW to further illustrate PM's point, In
Star Trek it apparently takes four Andorians to create offspring, but no one has ever explained how that works either. I don't see a large number of threads asking how and why here.
I'd also look to Earth species too - there are fish and trees that swap genders based upon environmental factors, and you have insects with Drones, Queens and Workers.