Greg is in denial, incapable of admitting he's an animal. There's nothing more disgusting, in his mind, than being a filthy creature who thinks it's better than all the other filthy creatures solely because it is capable of thinking at a higher level. Part of the defense mechanism that comes along with this stressor is that he believes he has no needs. Only animals have needs, so if he has none, it surely means he's not an animal, or so the logic goes. Autumn, meanwhile, is a healthy, well-adjusted, stable animal. She has needs, and has no problem sharing what those needs are.  Greg thinks Autumn is gross.