I see this as being every bit as requesting a male assistant in a gaming shop or electronics store. It's sexist.
You see someone who has to open her up and touch her insides as someone selling me electronics?
It not that operating on a cat is the same as selling someone electronics. What is the same is the utter irrelevance of gender in both cases.
As others have said, it would be different if the cat was going to be awake, and was a cat that was freaked out by one gender or the other (our female cat loves men, doesn't like women). I think it would be different, as a bit more of a stretch, if it was a human (either yourself or, say, your child). But I'm afraid I think it would be extremely rude, and insulting, to say that a male vet is not an appropriate person to operate on your cat. To me, that would both be excessively anthropomorphising the cat (which I know we are all guilty of from time to time) and also someone implying something distasteful about the male vet!
I'm glad the situation was resolved easily. In future, another's have suggested, I think it would be best to ask for the female vet by name, rather than imply that the male vet was somehow inappropriate.