|See also:||Mary Sue, Self Insertion|
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Wish fulfillment is something many fans (and "professional" creators) employ. It is using the creator's control to create fic, fanart, and other fanworks which address one or more outcomes that the creator wishes would come about.
The term in itself is a neutral one. "Wish fulfillment," however, is rarely talked about or mentioned as a positive thing but rather as a self-indulgent tendency that ignores good plotting, realistic characterizations, and descriptions.
Many fans use it interchangeably with the term Mary Sue. Other fans stress that it is only poorly-created, or obvious, wish fulfillment fanworks that they deem to be Mary Sues.
Examples of Use
Some people use Mary Sue to indicate a character that is 'too perfect.' Others still use it in terms of a character in a story that is a blatant or obvious wish fulfillment story; in other words a character acting out a fantasy of the author's. 
In the m/m relationships by female writers, I see an emphasis placed on both sexuality and sensuality, commitment and love. It's the best of both worlds, and probably unrealistic as hell, but I don't read Harlequins or write '/' fiction (the two genres are not otherwise, unfortunately, connected) for realism, but for wish fulfillment and fun. 
Mary Sue" (personal fantasy/wish fulfillment) stories will not be accepted unless they display some redeeming value. 
There is a very thin line between wishful fantasizing and True Believing. It's all too easy to cross that line when a fan's hopes for her favorite character have been dashed (as [M K] so brilliantly points out) and that fan lets her anger take over. It's a lot more fun and a lot more peaceful to simply patch up that kind of disappointment by writing the story the way you want to satisfy yourself. Or simply make it up in your own mind, if you don't write. But remember that this is the way that you want it, not Uncle George. 
Certainly, the B/D stories I have read have little to recommend them; they were never edited, & are patently private wish-fulfillment, plotless & colorless. B/D fandom's only virtue is its newness. It too will become dull in time. Fandom gets what it deserves.
I read '/' fiction, and for the most part find it pretty terrible. Hardly any of it is well written, or convincing. Most of it is just an excuse for fantasy wish fulfillment and a string of torrid sex scenes -- which make me wonder whether the writers know any thing about homosexuality. It's all so romantic. Yuk. 
I guess I write because I have something to say, and because I have ideas that I want to see explored. If some other writer was producing all the stuff I want to read I wouldn't have to write B/D. Isn't fandom mainly wish fulfillment, after all? Writers write what they want to read, particularly within fandom, and non-writers have to make do with what comes down the pike. 
By now it must be obvious that slash readers include women of all gender preferences. A more universal form of your question about why lesbians would want to read about men is, why should anyone want to read about characters who aren't anything they could ever be, and would actively dislike in life? Why do we read (with relish) about space pirates, neurotic rock stars, or melancholy Danish princes? Fiction isn't about reasonable wish-fulfillment or simple identity matches. Why should any of us watch Professionals, starring as it does two macho-prick studs? 
Is the majority of fanfic trash? Yes, it is. Cliched to the extreme, grammatically incorrect and, sometimes, simply and generically ROTTEN. Many stories smack of wish-fulfillment, personal hurts and insecurities related to life, work or sex. Some appear to have been compiled merely to provide a textbook example for the heading: STORIES WRITTEN BY THE UTTERLY CLUELESS. But is it without merit? No, because once again, writing is writing and to learn, one must DO. 
[With slash] the female, heterosexual reader does not have to mentally compare her own perceived shortcomings with either character. She can immerse herself (again, pardon the terms) in either (or both) of these hot, sexy characters having mind boggling intercourse with a beautiful, sensitive, intelligent man without even having to subliminally acknowledge that this does not and will not occur in real life. That is the essence of true fantasy. A lifting out of oneself. A lovely and uplifting moment of wish fulfillment. I am gorgeous and strong and heroic and loved, without any of that nasty backpack of self-perceived faults that might intrude if the situation were in any way comparable to 'real life.' 
...slash, too, is wish-fulfillment, because for many of us, the smarm doesn't go far enough! We want the men to acknowledge the depths of their attraction, and our attraction to the buddy pair, and so we bring in RL rules, one of which is that men in RL are either platonic friends, or they are lovers (who may be friends), but they aren't buddies. And slashers prefer to make them lovers, because hey--it's sexy and it's real, and for me, at least, that's a real kink. 
It's easier with fan fiction, since the characters are defined in their behaviour and relationships already. With new characters it can be a little more tricky to make them unique, because there is no model to guide us. This leads to a common prejudice, namely that any new character introduced in a fanfic must automatically be a self-insertion of the author.
Freud believed that all literature was in its basis self-insertion and all plots were wish-fulfillment. The only difference, according to him, lies in how well the author can mask it and gloss it over with redeeming social values. As a very broad generalization that may not be incorrect.If all characters are in some small way self-insertions, then what we commonly call self-insertion is only the extreme end of something normal and usually positive in writing. It is not different from "good" writing by nature, but rather by degree. So why does blatant self-insertion get such negative reactions? 
I just finished reading their (Sondra Marshak and Myrna Culbreath) Triangle -- in part because someone here mentioned that they liked it, and I enjoyed it enormously. Sure, it was obvious that this was the sort of Trek book that fan reviewers (especially fan reviewers who don't like fanfic) would have torn to pieces as "femmefan wish fulfillment." But I found it enormously fun, like a trashy romance novel or a soap opera. 
FYI, the ultimate piece of 'Mary Sue' fanfic is the terrific novel "The Beekeeper's Apprentice" by Laurie R. King, in which a young brilliant fifteen year old American girl becomes best buddies and eventually the wife of Sherlock Holmes (yes, really). King is an Edgar Award winner, which is the annual award given for best mystery novels. In "Beekeeper's" she doesn't just fill the 'Mary Sue' trappings, she exploits them shamelessly, subverts them, rises beyond them. She deftly points out the ways in which her modern female protagonist is no more a wish-fulfillment for women than Holmes himself was an idealized wish-fulfillment for Victorian men. For every allusion in the Conan Doyle canon to Holmes as a Christ-like figure (Reichenbach, his birthday, etc.), King counters with her protagonist's associations with Biblical women (both of the Mary's and Judith, in particular). 
Sometimes it's a bit of a wish-fulfillment (the writer putting herself in the position of the seduced character), but I wonder if it isn't just as often the writer seducing the focal character by proxy? For a woman (especially a young woman, which it usually is writing these rather immature type of stories) it can feel very empowering. 
Scully dated chavinistic [sic] morons. I can see why you are bothered by this, and you are not wrong to feel this way. At all. A lot of us back then dated them. We even wanted to have sex with them (head desk). Lucky Scully. She gets to dump the moron and both work and play with Mulder in this story, a double helping of wish fulfillment for women of the nineties. And, as a friend said to me in chat last night as we were discussing the story, at least the guy isn't a newly divorced, unemployed telemarketer/stock broker with anger issues and a homicidal tattoo. 
I [J.K. Rowling] wrote the Hermione/Ron relationship as a form of wish fulfillment. That’s how it was conceived, really. For reasons that have very little to do with literature and far more to do with me clinging to the plot as I first imagined it, Hermione with Ron. [...] It was a choice I made for very personal reasons, not for reasons of credibility. Am I breaking people’s hearts by saying this? I hope not. [...] 
... many women who enjoy slash fiction are also feeding, to an extent, their own internalised misogyny, here. We shouldn’t need to cast ourselves, in imagination or otherwise, as either a male partner in a slash pairing, or as a voyeur to a pair of lovers who are both men, in order to feel that sort of security. For those who have had life experiences which make them uncomfortable with a female in the spotlight being sexual (be it years of seeing glimpses of pornography and feeling ill at how objectified and gross it can seem to young girls only just figuring out their sexuality, years of advertisements featuring photoshopped female bodies which inspire only intimidation and feelings of inadequacy in so many women, or just the constant comparison/competition encouraged between women by media and culture to rank themselves against supermodels and their peers alike so that any female body in range becomes “competition”) it’s good that we have places we can explore sexual and romantic themes for our own pleasure alone, both parties of a slash ship being very attractive and enticing and, them being focused on one another, not having any expectations or pressures to exert upon the invisible voyeur that the reader becomes in such fiction. But… well…" [Note: this is only an excerpt from a very long and detailed essay. To read the entire work go here. 
...I can’t normally read het romances, a little dabbled into a greater plot is fine, but a pure romance is a hell no. I can’t watch rom-coms or romantic movies that way and for the same reason:
I will never have what those women have. I cannot be that woman. Comparatively, I will always be lesser, have been told I will always be lesser, and these things make me acutely aware that I don’t measure up. So it hurts. It hurts to be reminded of all my failings and the fact that no one will ever see me as a romantic lead or even potential romance in someone’s life.With slash, I don’t have to worry about it... I literally cannot be that character, so it’s free of that burden and I can simply enjoy two people finding each other and finding something I never will.
... why do when women grow older oftentimes try to distance themselves from the kind of wish-fulfillment and indulgence, self-indulgence, that we might have been really into when we were…teenagers. 
- from Interstat #74
- from Hanky Panky #12
- from the editorial in Purple & Orange? #16
- from Southern Enclave #10
- from Not Tonight, Spock! #10
- from Mixed Doubles #6
- from Hatstand Express Interview with Fanny Adams
- from Barbara T, "Strange Tongues" in The Terra Nostra Underground #9
- from The Ordover Files
- from MacGeorge's Musings on the Nature of Slash
- from Delusions are Grander
- from Self-Insertion and Mary-Sue-ism
- comment by Chris at alt.startrek.creative.erotica.moderated, reviews of Triangle, a Star Trek probook
- Asparagirl, FYI, the ultimate piece of 'Mary Sue' fanfic Posted May 23, 2007. Accessed November 19, 2007.
- a comment at Girls who are boys who like boys to be girls...
- wendelah1, comment about an X-Files story, Recycled Virgins, at xf book club, March 2010
- J.K. Rowling and Emma Watson discuss Ron, Hermione, and Harry: The full interview. 07 February 2014.
- defenestration-and-more's tumblr post dated July 7, 2014; WebCite reference link.
- lokeanrampant's tumblr post dated July 9, 2014; WebCite reference link.
- See also fan critic porluciernagas, Why Is There So Much Slash? in Ladygeekgirl, November 12, 2013.
- from Fansplaining: What's the Deal with Wattpad?