Oh yeah, I do that all the time too.
As I no doubt have said here before, there were quite a few gay guys in my literature course at uni (and even a lot more women), and I came from a minority thing high school where the word 'potter' (something like 'fag' in English) was commonly used as a means to refer to each other. 
So I came to uni, and I had it happen all the freaking time that I'd be talking to a gay guy and say (about some random other person(s) ) "yeah, those guys are real fags".
Of course I only realized 2 seconds afterwards that the dude was/is gay. I've grown more careful with the years and now I only do it once every few months, but still. 
It's not just the gay guys that go "dude?", the women draw their conclusions too. I have a feeling that they think I'm a gaybasher or something, hah. They should see the sweet man-loving that gets passed around all the time on clankiller.
