My girlfriend gets called sir a lot. Every time a stranger refers to her as a guy (sir, buddy, "him"), we give each other a smirky look, and can't help but feel bad for the person once they realize their mistake, as they most often do once she opens her mouth.
There are still times, however, they just can't tell the difference, either because they don't care to look beyond a customer if they're serving us in some way, or they would rather assume she's my boyfriend than believe we're lesbians. I look "too straight" and she, apparently, must be a guy in a T-shirt and shortly coifed haircut. (I call it the lesbian triangle.)