I personally consider myself their mother. I take care of them. They're dependant on me for shelter, affection, food, and life as they know it. I may not have given birth to them but as surely as some human baby is adopted by a mother and father and *they're* considered to be the children of the adopting humans.. why on earth wouldn't I consider my cats my children? I love them every bit as much, and in some cases more than the humans I know. I love them as much as I love my son (albeit he doesn't live with me, he lives with his adopted parents). To me, making the distinction and driving it home that they are cats, qualifies them as "only animals". I think animals have as much right on this planet as humans do, actually, in some cases I think they're more responsible than humans, but that's for another thread... and I think when you adopt an animal and make yourself their sole support, you've made yourself their mother.
Maybe this makes me a crazy cat lady, and it may (although I suspect there are those here that completely understand), but my vet, my sister, my family, and my friends know better than to insinuate anything less of my fur children than that they deserve the same respect and love that my own son does. My vet refers to me in our office visit as Mommy, and I take that as a sign that he *gets* my relationship with them, and he's one of the very best and most emotionally involved vet (with the animals, of course) that I've ever encountered.
I think that knowing the difference between owner and mommy might just be one of the very most important qualities he has, because it means he knows us, and he knows them, and he knows exactly what their personalities are. It's a sure sign that he cares.
I will grant that it may not be the case with every vet that it is with mine, some may just do it to be cute.