I usually do not mind the human roleplays in-game, but I can understand why some people have such a problem with it. First off, yes, humans are mammals. However, what MMORPG doesn't have humans as a playable character? See where I'm getting at?
It is often not an every game thing to be able to play as an animal with no restrictions. Usually, animals in a game are prey for hunters, enemies, and pets. There are actually not that many MMOs, if at all, where you plat freely as a dog, cat, and so on. Since there are such a variety with human characters game-wise, it could be a reason why people do not like them. You can be a human anywhere. If you want to be an animal and only animal on an MMO, tough luck.