I'm hoping this was gonna change already, but just in case the developers weren't planning on it, let me state my reason why it should be added. First off, it'll add needed difficulty. Imagine catching a bunch of females of a small animal, but not finding a male or vice versa. You'd have to either put more effort or maybe just trade at a local village for it. In addition, it'll require more maintenance. Imagine laying a whole lot of roosters instead of chickens, your outcome is gonna be no eggs but a whole lot of meat.
Chickens to Roosters & Hens
Roosters would obviously not produce eggs, but they will be required for hens to reproduce. Hens will be as chickens are now and they will produce eggs with or without a hen as they do in real life. Hens should never stop producing eggs but should slow down when they reach the age of 5-7 years old. Also if animals do eventually roam around in fences, would be cool seeing all the hens follow the rooster around.
Hares to Hare Bucks & Does
As for hares everything will be the same except for the fact that you need a male and a female to reproduce. Not having one or the other will not make them reproduce.