A sexist term used to enforce the misogynistic ideals that female characters/authors shouldn't be allowed to fantasize or write anything along the lines of wish fulfillment. Its misogynistic qualities are exemplified in many ways, most notably being the fact that it's not a term dominated by the male counterpart despite existing in a patriarchal society, as well as the fact that the male counterpart is largely undecided upon in name and also undefined (see urban dictionary's Gary Stu
entry which has no definition but to say "A Male Mary Sue", and the Marty-Stu
entry which involves the "Mary Sue" definition to define it).
It's usually used on the whole to bully new authors out of writing female characters altogether, making the task seem so daunting to some that they now only write slash fictions with two male characters, also exemplifying the misogynistic qualities this term involves.
Fan Fiction Reader: Why don't we just call all bad/annoying characters "special snowflakes" instead of using a female name like mary-sue in a derogatory fashion?
Troll: You're writing a mary-sue to pair with the canon character you fat low life, it's pathetic and so are you!
Author: I'm so afraid of having my female character labeled as a mary-sue that I only write male characters!