People can be funny about their dogs - the saying love me, love my dog springs to mind....

Maybe you could frame it differently - say it's for the dog's protection because if there was to be an incident the dog would need to be registered as a dangerous dog or destroyed.

FWIW aggression towards other dogs doesn't mean that a dog is aggressive towards people. My dog goes utterly feral if she sees another yappy white dog and she is on a lead but she is very gentle with children (doesn't like men much).