On PC I generally assume male unless told otherwise. Even girly avatars don't change my mind all that much because gender-bending used to be so "trendy" here that I almost used to assume girly avatars/sigs were guys and vice versa. And honestly, even though I say I assume everyone's a guy, I use the term very... loosely. Gender really means almost nothing to me so even if I sort of think of someone as a boy, unless I actually get to know the person, I don't really think of them as a person at all anyway. THAT SOUNDS AWFUL WHEN I PUT IT LIKE THAT but I just mean I don't tend to differentiate people very much until I have reason to and I take an interest in them. Until then they're just kind of a non-descript person. I don't even think about gender unless it comes up in their post.
On other sites like Tumblr, I assume female until proven male. :P