the CheeseBurger backpack episode tells us something important. There were more gems (or still is). From this image it seems like all gems are female other than steven (he has half human so that may be the deal) the lunar sea spire was a gathering place for all so it seems that if there were (or are) males they could’ve put one in the image meaning that there musn’t be any (pure) male gems. I could be wrong though but i strongly that there is no male gems .
They are shapeshifters, maybe the female form was popular. Or, since they’re immortal, their race doesn’t need males (for procreation). Or maybe they’re basically whiptail lizards (http://en.wikipedia.org/wiki/Whiptail_lizard)