I think it depends on the level of hissing. My sister will sometimes hiss if she sees my cat misbehaving. But hissing to express overt hostility while talking about what filthy creatures they are? That gets you banned. I don't expect everyone to love, or even like my cat. If they don't want to pet or interact with her, that's fine too. But what did guy did was much beyond that.
Yeah, it wasn't just about the hissing. The dude doesn't have to love my cat. Or even like him. I have friends that don't care for cats. But they're not outright hostile to cats. And this wasn't like a little "hiss hiss" (I've done that with cats that have claws. My current guy had his claws taken away before he came to me). This was a full on HIIIIIIIIIIIISSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS complete with saliva flying far and wide, due to the strength of the hiss. And coupled with talking about how "filthy" cats are (I'm sorry, but most cats I know spend about a 1/3 of their awake time grooming), and talking about how black
cats are the worst of them all (cuz, ya know, the color of the cat totally means something...). No. Absolutely no.
And part of what made me so mad about it (I probably should have mentioned it before) is my little guy is super shy. This is not a get-in-your-face-love-me-for-I-am-CAT kind of cat. This is a bashful Hi-I-am-scared-and-insecure-and-you-seem-nice-but-I-am-scared kind of cat. When company comes over, he doesn't come up on to the couch unless invited.
ETA: here are a few photos of him http://www.etiquettehell.com/smf/index.php?topic=92788.0
. He is a giant scaredy cat.