Adultery.  Cheating.  Infidelity.  Going out for burgers when you’ve already got steak at home…
Whatever you want to call it, it’s the ultimate relationship taboo, and a touchy subject for most.  Some can’t stomach it because it’s happened to them, and others just don’t even like to think about it.  Me?  I kind of have a thing for taboo, touchy topics, so I don’t have a problem with it at all, and it’s not because I’ve never been there.  I’ve been cheated on and, I’m ashamed to admit, that I’ve even been the cheater.  In either case, not that it excuses my behavior or that of the guy in question, the relationships weren’t serious.  Doesn’t matter to your self-worth or my conscience, but I digress.
As much as people dislike a cheater in real life, in the fandom Cheaterward is liked even less…unless he’s cheating on someone else with Bella.  This is, according to an informal poll I took, generally found to be much more acceptable than Edward cheating on Bella.  Okay, that’s a little fucked up, but consider the source.  Ours is a fandom that is based on the a book in which Edward was supposed to be a near-perfect man…er, vampire.
Here’s what the fandom had to say about it…

From a purely romantic, fairytale perspective, Edward cheating on Bella is NEVER acceptable to my emotional sensibilities, though if E is in a loveless marriage my heart can forgive him for cheating with his “soulmate”.  That is just twisted and wrong from a real life perspective and makes me laugh at myself for being so illogical!
-Anonymous

In Twific, because Edward is an idealized idea of men and because he is written by women, he very often gets away with a lot– except cheating.  Almost every action of Edward’s in any fanfiction story is at some point justified because he does it out of love for Bella. This is why cheating on Bella is the exception to this rule; because it is clearly not out of love for her, and this is why it is so rare to read about an Edward cheating on B.
-WhatsMyNomDePlume

I’m a canon pairings girl, so as much as I hate to say it, it often pains me to see these characters paired with anyone else. My irrational brain will justify pretty much anything to see Bella and Edward together – just about.
-Phoenixhunter47

Well obviously Edward cheating on someone else with Bella as E&B are just meant to be!!! LOL
-Anonymous

Edward is on such a tall pedestal, that most would rather see Bella cheat on him, than for him to cheat on her.  Not to mention, a lot of people just don’t think her whiny ass deserves him.

… In fic I would say that all cheating, even when E/B are cheating together is tough for me to take, but it’s not a deal breaker.  It is honestly harder for me to read E cheating on B, than the reverse, but even B doing the cheating breaks my heart.  I will really only stick with a cheating fic if it’s written with truth and remorse and change. I do not read cheating fics that are just glorified cheat/smut stories.
-RedVelvetHeaven

Now, I’m not pro-cheating in any way, shape, or form, but on two separate levels, I can’t stay seem to stay away from a well written Cheaterward fic.
From A Writers Perspective
As someone who writes, and writes a fic where there is cheating – of a sort, I find the topic to be rife with many compelling opportunities for a story.  There are so many different avenues to explore, and relationship dynamics to play with; also, you have the full spectrum of human emotions to work with, and I find the human condition to be extremely fascinating.  I think this topic necessitates that a writer really develop their characters, and the reasons behind the infidelity.  Otherwise, it can be extremely difficult – or in the case of Edward cheating on Bella, next to impossible –  for the reader to accept a reconciliation?
The problem is that there’s a decided lack of well-written fics covering about infidelity.  Most of them that I’ve found are fairly awful, and make it seem as if the writers lack even the smallest understanding of the far-reaching consequences, and devastating aftereffects cheating causes.  The few well-written fics out there, the ones that look honestly at infidelity, tend to be Bella cheating on Edward, or B/E cheating on others with one another.  Of those fics, I can only think of one fic that handled the topic realistically, yet with aplomb and grace, and that would absolutely have to be a fic called Comeuppance.
I was curious as to why a writer would choose to write a fic on this topic, knowing how well they are usually received, so I asked the writer, goldenmeadow, why she wrote Comeuppance.

When I got the idea for C/U, I had little snippets of dialogue and sexy passages that I knew I wanted to use, and these were between Bella and Jacob.  I’d just started writing fanfic, beginning with canon, and wanted to branch out into something different; believe it or not, I originally intended C/U to be a comedy (WTF? Yeah, I know).   Obviously it didn’t turn out that way, although I think there are some humorous moments within it.

What did you hope to achieve with it?
The best possible result in writing C/U has been the reader’s reactions, chapter by chapter.  To read the change in reviews from hating the main character, to feeling a certain complicity with her, to wanting to console Edward, to disapproving of his actions, to want them both to forgive each other, and then at the end recognizing that human love is a frail thing.  But even when destroyed, it can be put back together.  Not quite the way it was before, but perhaps more meaningfully, more realistically.

It was important for me to paint these characters as not quite perfect.  Because who among us is?  But also to allow the reader enough leniency to forgive Edward and Bella for their equal injustices and injuries…just as the two eventually forgave each other.  I suppose I wanted to take Edward and Bella’s epic, ‘high up on a pedestal’ love and bring it right down to the basest human level, but still make them our recognizable Edward and Bella (does that make sense or do I just sound like a pontificating asshole?).

I had a lot of reactions in which readers said they could now understand why one might cheat, and that it truly isn’t a black and white issue at heart.  I also had many people tell me they felt so much throughout it–that’s the most fuckin’ fantastic thing!

From a Reader’s Perspective
As a fanfic reader, Cheaterward fics appeal to me for a variety of reasons.  The biggest reason, is the same one behind why most of us read in the first place.  It’s a form of escapism, and allows you to experience things you otherwise wouldn’t have.  I won’t lie and say that I’ve never thought about cheating before.
I’m twenty-seven years old, and crazy about my boyfriend of nine and a half years, but I sometimes find myself thinking, ‘I’m never going to have sex with anyone other than him again.  I’m never going to feel the anticipation or thrill of exploring and being explored by someone new.  I’m never going to date again.  I’ll never again be surrounded by an air of mystery.  No one is going to try to unravel my secrets, because he knows them all.’
Then I usually panic, and start reciting the lyrics to Pink Floyd’s Time…and then one day you find ten years have got behind you, no one told you when to run, you missed the starting gun…wondering if that’s going to be me in ten or fifteen or even twenty years.

Reading about it lets cheating vicariously experience all of the positives – the excitement of being with someone new, committing such a forbidden act, the thrill of getting caught – and no one gets hurt.  If the story is more than just a gratuitously smutty story, you also get a reminder of what can happen when you cheat.
Which brings us to my next point of enticement: masochistic voyeurism.  I find adultery equally disgusting and intriguing, and the moment that the cheater is finally caught to be horrifying, yet mesmerizing.  You know it’s coming, and you’ve come to care about the characters, so you know it’s going to be painful.  Your stomach clenches, your respiratory rate increases, your heart starts pounding in your chest, but you just can’t seem to look away from the train wreck about to take place, and then it happens and you’re gutted.
One of the most jaw-dropping, stomach-clenching, heart-wrenching, can’t-look-away moments I’ve read in fic occurred in a slightly controversial fic called Caravaggio by MeiMei42.  The scene in which the moment of infidelity happened caused quite a stir, some people even publicly flouncing because of it, but I happen to have loved it.  Most people seemed to feel that it was unexpected, but I saw it coming from the start.  I just never imagined that it would happen in the spectacular fashion it did.

As we approached the bathrooms, I could hear noises coming from the men’s room, noises that were clearly the sounds of two people getting it on. I stopped in my tracks, and I could see Alice wore a petrified look. I moved towards the men’s room.
“Bella, don’t,” she pleaded.
I shoved the door open and stepped inside. There, across the bathroom, was Heidi shoved up against the wall, and Edward with his pants around his ankles, pushing into her at an angry and determined speed. Heidi’s eyes opened when the door did and, for a second, our eyes met. She smiled and I felt bile rise up in my throat. A hand grabbed my arm, yanking me out of the bathroom, and I stumbled back, stunned and dazed for a moment.
“Bella…” Alice whispered.

“Take me home,” I said, barely able to find my voice.

Ouch!  I mean…just ouch!
In this specific instance, I don’t agree that it was cheating per se, because Edward had – on numerous occasions – made it very clear that they weren’t in a relationship.  That doesn’t mean Bella was hurt any less, and the manner in which he did it – while they were out together, essentially on a date – was still so cruel that…I don’t even have words for how badly that would hurt.  The first time I read it, my jaw hit the floor and thinking to myself, ‘That was brutal and devastating,  The way in which it was executed – in a straight forward, non-superfluous manner, was really kind of perfect.’  I’m aware most of you won’t agree with me on that point, and that’s cool.
Infidelity will always be a divisive topic.  There will always be people who choose to badmouth a story – regardless of the quality – or flounce a fic simply because someone at some point is unfaithful, and no one is ever going to change that fact.
At  their worst, Cheaterward fics are ridiculous pieces full of drivel and bad lemons that read as if they were written by tweens with no understanding of relationships, and no concept of what they are even writing about.  Somewhere in between, they are usually a little from column A, and a little from column B, almost always an excuse for writing smut while pretending that it’s more than just porn with plot and, at times, they even have the potential to be entertaining.
Hate them or love them, at their best, Cheaterward fics aren’t really about cheating.  They’re about something bigger than that.

They’re about betrayal or survival or healing or consequences or loss.  They can teach you about love and forgiveness, they can reveal reserves of strength you didn’t even know you had, or show you the depths of regret  They can be stories about true love, and teach you who you really are.
When they are amazing though, they can tear you down, and build you back up, all at once, and leave you better off for having experienced it.

Comeuppance by goldenmeadow
If It’s the Beaches by Emibella
Caravaggio by MeiMei42