User blog comment:Lordofmonsterisland/Covenant Females/@comment-1515581-20100402043631

gender dimorphism could possibly be a very rare thing as Humans and a only a few dozen other species on Earth have this. It's possible that there is very little difference between male and female covenant races aside from esxual organs (not counting Jackals and Sangheili).

On another note, I saw an earlier post by Tuckerscreator who wondered why the Kig-yar would want to pick their gender (or atleast that's how I read it). In some reptilian species on Earth, a reptile can change it's gender to fit the needs of the species and keep their numbers up.