As a female gamer myself, I really can't get on-board with this, because I do it myself - I generally assume everyone else is a guy until evidence otherwise presents itself. Why? Experience, that's why!
Didn't occur to me it would offend anyone, because I myself don't get offended by it. I do swear a lot though so I do understand when people think I'm a guy, I'm not exactly very lady-like at the best of times >_>