Gender assignment is automatic when you picture someone in your mind, I would guess. I tend to attach a mental picture almost immediately when reading posts, and that means a gender gets assigned, too.

Why we choose the genders we do when creating that mental picture, I don't know. Sometimes it's based on the obvious (the genders of Jerome and Fran, for instance) or an obvious association (Nanne, I thought you were female for the exact reason you listed until I saw meetup photos; kcurie's name reminded me of Marie Curie until I saw numerous posts referring to "him"). Sometimes it's not so clear-cut-- I don't know why, but for some reason I thought In Wales was male for the first couple of weeks I was here.