Maybe cause you live in a city without a large gay population. Personally Ive been hit on a few times, I just smile say thanks but Im not gay. Gay man have extremely high standards, think of it like this... Girls are always saying too bad he's gay, just jump up and say Im not gay and he hit on me!
I dunno it never really bothered me... Who cares its not like he hit on you by grabbing your crotch and asking if you wanted a go.