Mine's probably not so much sexism, but a figure of authority for the lack of better description.

I work in a fire department, and I work in the front end. Oftentimes a citizen will walk in wanting a stamp for how many people can be in their restaurant. I'm the one who can help them, yet they inevitably look beyond me for a guy in a uniform (they're not trained to do what I do). It's annoying, but if they want the stamp, they have to deal with me, so eventually they'll get it.

I was once at a clothing store in a mall. It was a female-clothing store, so the lone guy in it kind of stood out. I guess it's really no different than seeing a girl at a bike store, but it takes a moment to shift your brain into gear. It's great you're working at a bike store, though. I once aspired to become a bike mechanic, but I got steered into clerical work. I guess I can still learn on the side!