I don't want my roommate to get a cat.
Are you afraid of cats? Are you allergic?
I don't like cats and I can't make an exception for this one.
I understand that you don't like them, but when you share a living space then you should be considerate of your roommates too.
It will be annoying and dirty, I don't want to live like that.
Cats are very clean pets, and I think if you just give it a chance you could really learn to love cats.