In as non-hostile a way as possible - who are you to tell them what their job is?
I do cat adoptions. I have refused adoptions both for people having unaltered animals in the home, and for the intent to declaw (I was willing to adopt an adult, already de-clawed animal to them, but they wanted a kitten. Kittens scratch. Can't deal? Get a fish.)
My JOB is to find GOOD, PERMANENT homes, not to send the animals out the door with the first person who can fill out an application. These animals' future rests in my hands. They have no say in who they go home with. I have to sit with someone for 10 minutes, and try to glean from a piece of paper and a few questions whether or not this person(s) is going to give this animal a happy, healthy home.
As for the OP, the fact is, most people who don't want to alter their animal are pretty sure THEY are the ones who have a GOOD reason not to, and that it's everyone else that is really the problem. Everyone's got their reason. That's great. Not my problem. My problem is that 70% of the animals that enter a shelter are euthanized because there are too many animals. There are too many animals because so many people have reasons not to alter their pet. Maybe you are the exception. Or maybe you just haven't had the misfortune to become one of the statistics. Either way, the adoption counselors rarely have the time or inclination to sort it all out. And yes, I'd be concerned about aggression issues, too. Again, everyone is sure THEIR dog will be the exception. THEIR dog is sweet, not aggressive! Well, maybe you're right. But again, from the perspective of the adoption counselor, it's a risky adoption, and your belief in not altering your current pet flies in the face of spay and neuter campaign that most shelters believe in with fierce desperation. Given that, you can't REALLY be all that surprised by their reaction, can you?