+8
I think I've said this somewhere before, but I tend to assume everyone online is a guy unless they have a username with a female identifier in it (like Miss Vicky for instance).
If someone has a female avatar that is really sexy looking, I assume it's a guy. But if they have a female avatar that is not sexualized, I might be more inclined to think that they're female. In either case, I'll use whatever pronoun I see other people using in relation to them until corrected.
I myself am female, but I completely understand why people would assume I'm a guy. I'm not exactly ladylike in behaviour (especially not IRL where I swear like a drunken sailor), and my tastes and hobbies are typically associated with being masculine.
It never bothers me when people think I'm male. If I was someone else I'd probably think I was male too.