All of us have wondered, but not many of us have taken the time to do the test. I made two pens with two pigs each. When I bred the pigs, I kept track of the gender of each and took out the piglets. I then destroyed the fencing between the two pens to conjoin them and let the pigs reach each other. After that I bred both female pigs and both male pigs. The two females had one piglet, and the males had one. Homosexuality in Minecraft animals still makes more of that animal, so I can only assume that during the apocalypse that led to the modern Minecraft, radiation or experiments or mutations of some sort led to all the minecraft animals becoming more like frogs, which can change gender if needed.