A straight guy who works at a mixed (gay and straight) bar in my town seems friendly on the surface, but when he's holding court over some of his straight friends he makes generalizations about gays that, frankly, make my blood boil. (I don't think he knows what my orientation is because I'm not stereotypical.) Gay men are this, gay men are that. I wouldn't necessarily say he's outright homophobic, but it's like he ignores the realities and diversities of the gay men who come into the bar and spews out these same old stereotypical notions. What do I do about it? Anon.
Say something. But first, go into the bar with a male date and make out at the bar right in front of this schmo. That way he'll know he'd better watch his step around you and watch what he says. Then the next time he says something stupid, definitely call him on it. Or say something before then. It's a perfect opportunity to educate someone. If he's an intelligent person, he can learn from what you have to say. If he's an asshole -- and he just might be -- then you're wasting your breath. But still try it. If this is a bar that welcomes gays and takes their money, why should you have to put up with his dumb remarks?
I can relate to what you say that even though he works in a bar with many gay customers he only sees what he wants to see. Some straight-identified men are only comfortable with gay men who fit into their narrow notions of what it means to be a gay man. It's like the rest of us are invisible. It's similar to white racists who only see African-Americans in limited terms, and can't see the diversity of that community even when it's staring them in the face.
Lastly, even people who seem gay-friendly can be homophobic or have some outdated, homophobic notions. Don't simmer in silence. Confront them on it. It may make for an unpleasant experience if the person you confront is a jerk, but maybe you'll get a pleasant surprise.
In any case, don't let him get away with it.