Humans are mammals, and like most mammals, a greater part of the reproductive burden will fall on women. That’s not fair, but nature isn’t fair.
Well, actually in most sexual species the females invest more energy after the reproduction, and the males invest more energy before the reproduction by competing to mate (even to death), there is a balance there if the term is more suitable than "justice". Besides, Among the social mammals while the females invest energy taking care of the offspring the males take care of the territory and the access to resources.
