If you guys are young, I'd just correct them and say you're a guy. Why? My son used to have really long hair (shorter now). And even though he dressed like a skater, people would call him she, bascially because of his hair. Usually he'd just give me an eye roll, but sometimes he'd say I'm a boy. He'd say, what, are they dumb? I'm wearing boy's clothes!! because it doesn't occur to him that girls might wear the same thing (I haven't seen it, or if I have the girls did have a bit more color in their outfits). Most people would say oh, sorry, when corrected.
So that might work if you are younger. We tend to be so self conscious because we're trying to pass, then get upset when people see the wrong gender, but in some cases if you correct them they will be like oh, okay, and not immediately think ooh, transguy.
Jay