I know this is a little far fetched, so it may work if you could turn it on and off in the settings or in the sandbox thingy. Perhaps there could be a mutation for your nichlings kit to be either female or male. The genes would both contribute a small chance of that gender, and would be very hard to unlock. One idea for unlocking it would be making it to home island when playing story mode, and it unlocks it permanently for both that tribe and tribes on other save files. An alternate way wo