This may be an unwarranted, "third answer," but personally, if I were crossplaying, and someone believed I was of the opposite gender, I'd be pretty amused...and perhaps a bit proud that I was able to assemble a convincing look/outfit/cosplay/(Crossplay?). >_> But then, that's just how I'd take it.
But anyway - if a person makes a deliberate choice to try appear as a character of a different gender than their own, I don't think it's that reasonable to be offended if someone mistakes their gender. I mean, it could be that they just look convincingly like the oppisite gender, the way that they've dressed-up.
Although, if the person who mistakes their gender is twisting their comment around to be an insult to the crossplayer, then that's another story.
I think I'm being gloriously redundant, tonight.