Humans are restricted by gender if they want to copulate and conceive. That's not true for we Trolls. Since we do not lay the eggs that hatch grubs, gender really is more of a preference than a need.

Also humans, for the most part, can only have one at a time and the female is generally incapacitated for months. Then the human parents get stuck rearing said offspring instead of going off and being productive. Since the Mother Grub lays an untold amount of eggs, no Troll has to essentially be nonfunctional for any period of time. Adults also do not raise young grubs at all, and thus their energies are better spent in the Fleet.