If "gaydar" exists, I think it's an instictual sense. It's not observation of set stereotypes. I say this because a man can be very heterosexual in appearance and behaviour but, if he's gay, I will pick up on it, often before he does. They say men and women give off a vibe or an undectible scent to attract the opposite sex. If you believe in that then maybe gay men do also without knowing it. It's part of the base animal instinct, evolution, the ability to find one's mate, etc.
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