Because they might not be.
I found out today that one of my friends is gay. I'm perfectly supportive and an equal rights activist. I was just surprised. I sincerely thought that his flamboyent sense of humor was just that - offensive joking. (Is it still offensive for him to joke like that?) It's like an African-American teling racist jokes.
I found out from McKenzie.
"...and Jeff's gay."
"Is he? I thought he said something about a girlfriend."
"No." She replied, "he means girlfriend in the same way I might." She sounded as if she was laughing at me for not noticing.
"Oh, OK. Whatever." I shrugged. "Whatever" is not usually as sincere as I put it.
I tend not to assume that someone is gay until I know for sure. The lisp didn't tip me off. After all, why should it have? But was assuming that he was straight just as offensive as assuming someone is gay because they fit a stereotype?
The bottom line is you can't just assume anything. Assuming can get you in trouble.