"Actually, I was just looking for a seat, not a relationship". :-D
Although, I've run into similar situations once or twice at work, places where I'm just casually out (ie, I don't make any effort to hide it, but don't broadcast it either). A female co-worker will ask if I want to grab a coffee or something and I wonder if I should say something like "You know I'm gay, right?" Which could invite a response of "it's only coffee, I wasn't proposing a date".