It happens to me constantly, and it frustrated me all through school particularly. The odd thing is, that no Gay people ever thought I was Gay. It was ONLY NT straight men who decided they somehow figured that. I have always worn gender-correct clothes, and always been into male hobbies and interests (cars, motorcycles, etc.), and yet I have straight friends who never were accused of being Gay, but who's hobby is cooking. Luckily I grew up in an area where there wasn't much violence or hatred against Gay people, or I'd have been in trouble, I guess.
I still have no idea why anyone would think I was Gay, when I never expressed any interest in other men, and it boggles my mind as to why any straight man would even think about another man's sexual interests in the first place. I guess it's just human nature. It's almost gratifying when I find old classmates on Facebook, and they act astonished that I'm married to a female. Isn't that a big fat surprise! . Just for the record, I'm usually more comfortable around Gay men that straight, when in a group. Somehow I don't need to keep my guard up as much, if that makes any sense.
The only thing I can come up with among us as Aspies and High-Functioning Autistics, might be the absence of body language. I guess NT males give off certain body language that indicates sexual preference, as they say Gay men can spot a fellow Gay man a mile away when NTs can't, and vice-versa.
Charles