I know it's a real thing, but I don't have it. At all. Even stereotypically gay clothing and behaviors do not cause me to make judgements about someone's sexuality without direct evidence. A lot of people assume everyone is straight until proven otherwise. I just assume that everybody's pansexual until they tell me different. The world looks a lot hotter that way, anyway.
I think not having functional gaydar is because I'm bi, to an extent, but more because I recognize that I tend a bit towards androgyny. I dissociate my typically-feminine and -masculine traits from my own sexuality, so I naturally don't draw conclusions about other peoples' sexuality from those cues, either.
When I came out to an ex as bi, he said, "Yeah, I figured that out years ago, but I didn't say anything because I didn't know if you knew."
Pedro has incredible gaydar. He can even tell with reasonable certainty the difference between a lesbian and a bisexual woman in a relationship with a woman just by having a five minute conversation (not about relationships or sex). He says there are subtle nonverbal cues in the way a person responds to him; if that person is attracted to men in general, s/he will treat him as a potential sex partner even if s/he's in no way attracted to him specifically. He says it's so ingrained that it's impossible to turn off. I've never seen it.
He also has the ability to tell almost instantly when he sees someone on the autism spectrum. His biggest clue is when someone doesn't know what to do with his hands when he's walking, but on occasion he's been accurate from still photographs, too. He's a great reader of people and subtle differences really stand out to him.
I, being probably somewhere on the autism spectrum myself, am a terrible people-reader. So it's good that I have someone else around who is good at that kind of thing to help me in social situations. This is why my social group includes a woman I've known since high school, an exuberantly nerdy guy in his forties, a twentysomething married couple who are new to the area and painfully shy, and a thirtysomething guy with Asperger's. We have two things in common: we all love board games, and we all suck at being social. It really takes the pressure off.