It's true about Bella, but then--Edward is perfect and beautiful and angsty and tortured and all the girls want him and he is, like, a super awesome vampire, and he is just as self-centered as Bella, but no one calls him a Stu, you know?
Good point; Edward provides a particularly good example for this discussion.
My own definition of Sues centers on the author's or audience's wish-fulfillment. Going by this logic, I don't consider Edward a Sue because the author doesn't live her fantasies (or her readers' fantasies) through him. Instead, she lives them through Bella. Edward is certainly a one-dimensional character in this scenario (and borderline fashion accessory...) but not a Sue because he isn't the character that the audience--and author--identifies with. Edward's super-awesome-vampireness only makes him a desirable object of Bella's affections. Now, if Twilight's author was a male (or if male readers
wanted to behave like Edward), then he would qualify.
Of course, fiction provides wish-fulfillment all the time. For me, a character only turns into a Sue when he/she veers so far into "I win all the time and everybody loves me" territory that it becomes unbelievable and annoying.
If we assume that wish-fulfillment defines a Sue, then we can also explain a lot of the gender disparity quite simply: more females write fan fiction than males. In canons with a male audience (Evangelion comes to mind), male Sues emerge with alarming frequency, and are labeled as such. In canons like Narnia, fewer male Sues appear.
