Yes that's a thought knockedgymnast, he's on the scam in some way.
I think it's a horrendous liberty to take someone else's cat to the vet to be neutered. Just awful.
I would say though that male cats need to be neutered. My family have had many long lived cats since I was a child and it's essential to get a male cat neutered to avoid the stink of them spraying. They really do need a chip and/or collar with a bell on it. They'll still catch birds with it but if you do so it sends out a message to the society you're cohabiting with that you're being a responsible neighbour. Disgruntled old codgers will look for any excuse "That damn cat is spraying/digging up my soil/killing the lovely birds"
If you put the belled collar on the cat, chip him and give him a clean litter tray and fresh food and water daily, you'll have a better conscience and feel entirely entitled to be justfiably angry if somebody does this to your cat.
By the way, if you do the above, the cat will not be a 'tart' as one of the posters suggested. One of our cats was so loyal that she used to patiently wait for my father at the end of the street when he returned from work. The other cats would follow my parents if they walked to the nearby woodland. Sweet and not in the least bit tarty.
I wouldn't do the catflap thing, we used to live near a busy road and one lovely cat was runover. Since then, they're kept in after dusk. They don't mind at all if they have a fresh litter tray every day.
I'm very glad that you've sent a letter to this man. The very fact that he took the cat to the vet without your permisson says a lot about his emotional state of mind.
Beware - this type of person might play dirty. I think that you should get a solicitor to send him a letter and the vets should also get a letter too for not checking the ownership. What name did he use for your cat? Have you asked to see a copy of the bill? I would visit the vets in person with your husband.
As for it being wrong for the OP's husband to pay the criminal (for that's what he is) a visit, I disagree. I think that your husband's presence in the matter will ensure that he realises the gravity of the situation.
If you want to play equally dirty, you could claim that since this has been done, the cat hasn't been the same and has become very withdrawn and for days afterwards he was walking with a limp. In fact, I think that you should visit your own vet and take him for a check up and explain what has happened.
Somebody this devoid of emotion is capable of harming your cat again. I urge you to have him chipped and collared. I'm so sorry that you had to suffer this violation but next time get a male cat neutered.
Sorry for the long post but this has really angered me.