Don't feel annoyed by the assumption. I'm not. I'm just intrigued by it because I try to be inclusive in how I think and express myself. I always figure my cis-male-ness will shine through unconsciously anyway. When it doesn't I try to learn from what happened.

BTW, Hawk often is assumed to be a guy online. One, her name is seen as masculine— even though hawks, the wild predators, are both male and female. Two, in many forums, the default assumption of male-ness is supported by the fact men are the majority, sometimes the lopsided majority, of participants. And three, she eschews traditional gender norms. You won't see her writing about makeup or shoe shopping (except to complain occasionally how all but a few brands/styles are impractical). Though if you read Ask A Manager (the forum) a lot, you've almost certainly seen her write there.

Finally, I actually agree with your shorthand that anyone writing a blog about themselves— not strictly about external things like books, movies, or sports— is likely female. Writing (or speaking) about oneself requires a willingness to be vulnerable. Expressing vulnerability is very much not a traditional male trait. (I endured frequent finger-wagging about this from acquaintances and even family when I was younger.)