I think there are a myriad of reason a guy could be asked this question, some Aspie-related, some not.
For instance:
--Gay guys and dorks (i.e. someone not socially aware, and I mean this in the nicest way) sometimes dress similarly, in that a dork might wear something tight-fitting and not realize that it was inappropriate for straight men to dress this way. Case in point, I had a roommate that dressed like this and wasn't obviously gay, so I thought he was a dork. Turns out he was gay and only a little bit a dork (btw, I'm a big dork ).
--Aspies, not always being socially aware, might use gestures, language, or other forms of social communication that are outdated and now connote new meanings in today's society. For instance, the hand-pushing "Pshaw" movement from the early part of the 20th century (think "Little Rascals/Our Gang") has now become a stereotypically fey gesture in gay society.
--Any guy who seems on the surface to be normal, good-looking, attractive, etc., but does not have a GF or wife will eventually get noticed by others. People then want to know, is the person avoiding heterosexual relationships, and if so, why? It sometimes seems amazing to others that someone who looks so normal could have issues, internal ones, that stymie their attempts at a relationship. During my very long dry streak, at one point my physician was amazed to hear that I was involuntarily celibate. His first reaction was to send me to a shyness support group -- apparently he felt that I was way underperforming my potential, especially given that I actually wanted to be in a relationship.
--When guys are too pretty, which isn't always intentional, they will also be suspected as gay. When I was in my teens, I was more pretty than handsome. Women actually would tell me that I was too pretty at the moment, but that I would be a handsome older man (I hope they were right ). I have a coworker who everyone thinks is gay, just because his eyes are pretty. Sometimes people don't realize beauty could be entirely natural, and they suspect instead you are doing something to enhance it, which would likely be very gay (though there's nothing gay about trying to look more masculine/handsome, vs. pretty).
--If you are moody, pensive, emotional, etc., you may also be suspected of being gay. A lot of people aren't comfortable with men showing emotions other than joy, anger, or boredom. This is more society's problem than yours. That being said, once I got on hormonal therapy, some of this did get toned down.