It’s not the term itself, it’s how it’s used. And it’s sexist because no one ever accuses male characters of being a Mary Sue, or a Gary Stu, or Harry Flu, or whatever.
Wesley Crusher (a man… sort of) is quite possibly the most referenced Mary Sue character in the history of fiction. He is even mentioned in the Wiki page!
Well if it’s on Wiki it must be true.
I’ve literally never heard a single male character called this other than what you just said, and I’m not the only one.
And sexism takes many forms. You don’t have to be physically assaulted to be a victim.
I do not think Rey is a Mary Sue, but I will not just throw the accusation of sexism at people who think she is. I can understand why someone might say that she is a Mary Sue for seemingly mastering the force so quickly. I think since she had been living in the desert fending for herself for pretty much her whole life it makes perfect sense to me that she could handle herself and learn quickly. Being able to beat Kylo Ren and read his mind without any legitimate training is what bothered a lot of people.
I still think that complaining about a word and saying we should stop using it, especially a word used to label fictional (as in not real) characters, is the height of first world problems. The reason I brought up my example of sexism earlier was to implore people offended by the word “Mary Sue” to #CheckYourPrivilege.