IMHO, I think Gaydar is a result of trying to find signs in others that they are like us. Straight people generally don't have it because they assume everyone is straight. Gays, on the other hand, are more open to others being Gay like themselves and maybe even unconsciously look for characteristically gay movements, behaviors, tones of voice, eye movements, and other clues. Notice that many Gay men don't have Gaydar when it comes to lesbians (except, of course, for the Ellen DeGeneres type - which is a dead giveaway - and straights can pick those out too), because in general, our attention is searching for others that are Gay men, "like us."