I'm not sure I understand literary criticism (working from just a high school education, here), but when I think of Mary Sue, I think of someone taking a story with an extant history and *inserting* a viewpoint character/alter ego into it, in ways that distort the previously-existing relationships and power distributions. That is what distinguishes Mary Sueism from straight wish-fulfillment.

As for sexism, the reason it's Mary Sue is that the stories were made up about television shows that had all or mostly men as characters. (But you knew that, right?)