I can certainly agree that some people will find some "pets" species more charismatic than others. The magic though is in the eye, the lens and hence the photo. That's how I look at it. This is a photo contest after all. Also, now that we've opened the whole "if there's Cats, then there should be Dogs" can of worms, then I can see how pet owners of other species will feel left out too. Do we then have Fish, Snakes & Lizards, Mice, Roos... wait, or do we separate Snakes from Lizards? I think you can see where I'm going with this. Once the cat's out of the bag...
Once a few of you mentioned "pets" or "animals" as an alter theme, it just seemed more logical, more inclusive and more fair to all involved. No?