Thinking that Rey is a Mary Sue isn't inherently based on sexism or gender bias. It may be, but it may not be. Why assume that it is? Isn't that a way of just negating or ignoring what someone is saying? I mean, I get that there are actual sexist takes on this on the interwebs, but EN World is pretty mild in that department, and I like to think that as a community, we can be a bit more tolerant of people's views and not jump to such conclusions.
On the other hand, even if Rey is a Mary Sue--or rather, to what degree she may or may not be "Mary Sueish"--does not write her off as a good character. For one, Daisy Ridley is a better actress than Mark Hammill or Hayden Christiansen were in their respective roles. Secondly, let's remember the trajectory of the films, as far as the primary "Force hero" (or anti-hero): we go from Anakin to Luke to Rey. Why can't the third in the line be the most powerful?
Not to mention that if she is Mary Sueish, then Anakin and Luke are Gary Stuish. Maybe not all to the same degrees, but it is a matter of degree.
Finally, I think part of the Mary Sueness is based on cinematic context. The third trilogy was created after the MCU revolution, so it makes sense that--for better or worse--heroes are more super-heroic than in the previous two films. the prequels were made after Hollywood started integrating Chinese cinematic action, so Anakin and Obi-wan could do things that Luke couldn't (or didn't). Rey did things that were clearly influenced by MCU and enabled by improved technology.
Now even if there is some gender-related Mary Sueness (that is, propping Rey up as the Best Evar because the Future is Feminine, etc), given the long history of male heroes dominating--not to mention the fact that I am the father to two girls, so love seeing such strong female leads portrayed on film--I have no problem with it. I mean, so what? And this goes to both sides of the argument. If you hate the Mary Sue argument, it is likely because you either think it is inherently sexist--which it isn't--and/or that it is just 100% wrong, which seems far-fetched in that all such heroes have some degree of Mary Sue/Gary Stu-ness.