I consider it to be a bit of a complex question, moreso than presented.
"Under whot circumstances?"
If it's a casual relationship, and the partner is not providing whot's required of them, despite being told exactly whot's needed? Then I can rationally see seeking alternative sources for stimulation of that requirement.
For example, let's say if I had a mate who didn't care for my hobbies at all, didn't discuss anything on an intellectual basis in general, or just didn't talk as a whole, but had no problems with insisting I not be allowed to be friends with anyone of the opposite gender at all (that would reaaaaaally suck, most of my friends are guys), then we have a problem...
A relationship is always give and take, if yeu only give and recieve nothing, then it's a parasite. At that point most people will break things off... however, for the longest time, this was heavily frowned upon socially. This's why marriage divorce rate is so high now... people are marrying about as poorly as they always were, but they're not being FORCED socially to maintain it... no more excommunication from the church and being damned to hell for breaking up with a husband or wife who's abusive. This means alot of the marriages that USED to stick through the worst things now fall apart. Why stay married to someone who doesn't care?
Thing is, 'cheating' on someone was very common due to this... having a mistress etc was commonplace, at the height of rome's power, it was pretty much assumed to be the case. It was not uncommon at all for a roman male to have a female wife, yet have a male mistress. Why was this not so much a bad thing? To have a truly healthy relationship, several criteria must be required to be met, if yeu aren't recieving adequate provisioning, be it emotional, mental, sexual, and so on, then yeu're not going to care for that individual any longer, will eventually grow distant, and subsequently malicious towards the person yeu used to love.
Having someone who just doesn't make yeu feel loved, no matter whot else they provide, will fail, period. If yeu're an ENTP such as myself, then yeu'll have an abnormally high emphasis on intellectual stimulation as well, if I can't even talk to my mate and can't get them to hold an intelligent conversation... I learn nothing, I don't grow, and it feels like they just honestly don't care about whot I have to say, which then degrades into worse matters.
If yeu can find someone else to replace them for one or two sections of whot's required, then the relationship will generally be more healthy for it. People 'cheat' in many many many ways, NOT just sexually... let's say yeur husband CAN'T or WON'T provide yeu with emotional support of any kind... they're the kind of person that's just pretty much dead emotionally. If yeu need a hug, they'll hold yeu at arm's length. At this point, most people will find someone else to confide their problems in. If they don't, they'll just grow spiteful and eventually hate this husband who didn't care for them, eventually ending it entirely.
Now yeu may think "well that's good then they didn't belong togeather!"
But the thing is, NO RELATIONSHIP IS PERFECT!
Therefore, there WILL be situations where yeur partner will not be capable of providing for yeu in one way or another. At this point, yeu can either let it fester and grow worse, or yeu can fill in the gap in some way shape or form, and continue to care for them in all other respects.
The most common of these is whot I listed above, someone's spouse just won't listen to them, they just don't care anymore or never were able to, or just can't express themselves in the manner required. They can't show affection in the way the other individual NEEDS to have it. If that need is present and unfulfilled, it will usually be fulfilled instead by a close friend to confide in, and it's far from uncommon to have a woman in a relationship who just needs a hug sometimes, but their guy/girlfriend just can't provide, so they go to their best friend for consolation in rough times and get a hug there. It's not neccesarily sexual, and therefore usually not considered to be 'cheating', even though it's the EXACT same problem.
Why is sexual infidility considered so poorly? Taboo, mostly. An additional of personal pride as well I suppose. Honestly, I think people should be way more up in arms about the emotional level myself... if yeur spouse is finding emotional comfort elsewhere, yeu are NOT showing them the LOVE they require. If they only keep yeu around because yeu're good in bed, then seriously, yeu're little more than a hooker to them, so why are yeu feeling so smug and satisfied that they're not 'cheating' on yeu sexually? They ARE emotionally, which's arguably far more important...
The same goes for intellectual stimulation as well, especially in most NT's, we need to think, use our brains, bounce our ideas off people, ENT's exceptionally so as generally we absolutely must be able to relay our ideas to another person. If we don't, then it drives us mad and makes us feel unwanted.
Yet once again, having low 'mental fidelity' is not considered odd... it's actually considered perfectly acceptable to 'love' yeur chosen mate, but spend all yeur intellectual time with someone else entirely, debating and discussing. Why is this considered more acceptable?
And then we come to the sexual issue... and this's even more of a problem because no two people are perfectly aligned. Everyone, EVERYONE, has their own individual preferances, fetishes and so on. Maybe yeu get turned on when someone tickles yeur feet, maybe yeu prefer something a bit more bizzare that most people would find rather creepy, but there's always something that yeu prefer, it WILL change over time, same as yeur sense of taste... every few years yeur sense of taste changes (approx every 7 years), so that things yeu used to like yeu may no longer care for, while things yeu didn't care for yeu've grown fond of. I used to drink iced tea all the time, now I won't touch the stuff, same concept. Sexually, it's no different, well it may be faster or slower in its' changes, I haven't seen any studies on the estimated timeframe yet. But anyways, yeur tastes do slowly change over time, yeu get tired of one thing that yeu've done too much, bump into something else that yeu really hadn't given thought to before and find yeu may enjoy it, whotever. The point is that yeu will change in yeur sexual preferances over time, so will yeur partner, and eventually there WILL come a time where yeu'll diverge in some areas. Don't expect to be perfectly flawless at all times. Yeu will eventually develop some odd preferance they find is just weird and they won't be able to provide it. The same will happen in reverse. If this happens enough, eventually yeu may have issues with intimacy of a sexual nature as neither may be willing to stimulate the other in the way they so desire.
At this point, yeu have several options... yeu can leave, and abandon anything else they may have provided, yeu can ignore it and hope the problem fixes itself, but it'll probably take so long to change for the better, if it does at all, that yeu'll likely just end up leaving them anyway eventually, but in a far less favourable way as that taint begins to fester and break into other parts of the relationship... or yeu can find someone or something else to cover that which yeur partner no longer can, or never could to begin with. Sexual toys, pornography, erotic stories, etc, these are all designed to aid one's ability to cover things which they are not currently being provided. If yeur partner refuses to use those, then chances are yeu're going to break up. If they will NOT allow yeu to be satisfied, then that'll quickly turn into frustration and annoyance that they really don't care about yeu, and it spirals downwards from there.
So... yeu either end up cheating on them, which may or may not placate yeu, or may breed further resentment, especially if yeur partner doesn't agree with it, or it's kept in the dark. If it's hidden, then it'll probably go badly.
End result, is if yeu're not open about whot yeu need, and specific about whot yeu're not recieving, then yeu're going to fall apart anyway.
As such, 'low fidelity' is not really uncommon at all, it's pretty much existant in every single relationship at one point or another. If yeu have EVER told something to yeur best friend that yeu DIDN'T tell to yeur spouse, then yeu already have low emotional fidelity. If yeu discuss things elsewhere because yeur spouse either doesn't seem to care, is too busy, or just doesn't seem capable of grasping it, yeu have low intellectual fidelity. Sexual is whot is normally implied, but seriously, it's one of the least important of this group, yet it gets the most attention. This should be the least of yeur worries, as it's the easiest to fix via alternative aides available. Realize where yeur flaws are and try to compensate for them, don't just ignore them and hope they go away, because it'll mean that the one yeu cared about WILL go away.
In that line of reasoning, I do strive to maintain my fidelity as best I can, however, I'm also a realist... if my partner does not want to compromise or attempt to work out a problem, be it emotional, intellectual, or physical, then they have just essentially told me to "look elsewhere", and I will because they refuse to do so themselves. At that point it's no longer my problem, I tried to solve it, I presented plausible options and alternatives, they rejected everything, or didn't want to hear about it, and didn't want to help, so if they don't want to provide, then someone or something else will. I can't be made to feel guilty for their lack of caring, as it was their own refusal to act in any way at all to change things for the better. If that means I might end up doing some rather ...interesting... sexual stuff online, to be able to sate myself, then so be it. I'd really rather not, and would've far preferred to've done something with someone I trusted and cared deeply about, the emotional aspect is as important as the physical, but if yeu seriously can't seem to get any stimulation out of it, and they're not even trying, then it's their fault for not trying and they should've taken the opportunities they were given.
Of course, if yeu DON'T tell them about this, do NOT try to work through the problem, or they don't even realize that there is a problem... then yeah, yeur attempt to 'not hurt their feelings' just made it YEUR fault. Communication is everything. It's cliche and sounds dumb, but it's true. Yeu HAVE to tell them whot the problem is or they CAN'T fix it. They can't just read minds, they need to know whot's wrong, the whole "well if yeu don't know whot's wrong then I'm not going to tell yeu" is total crap and yeu know it. Yes, the mind games are fun, guilt tripping can be amusing, but the fact is yeu will never get whot yeu want if noone knows whot it is yeu want to give it to yeu. Making it into a russian roulette game of chance doesn't get yeu anything of value, yeu've got to actually say whot yeu want, and try to put some effort into getting it. If I want something, I take it, or do whotever it takes to recieve it. I've learned over time that I can't just go "oh well just get me whotever for my birthday" and expect to get whot I want. It's a retarded guessing game that doesn't solve anything, and I'm the one that ends up loosing in the end because I didn't just come out and say whot I wanted. I don't do this anymore, or at least try really hard not to...
But anyways, my point is that fidelity is not as important as people make it sound; the act of NOT SAYING whot the problem is and expecting yeur spouse/significant other to just 'magically' know whot yeu want? Yeah, that's total crap and yeu know it. Tell them whot's wrong, work towards a solution, and yeu'll both be happier. If yeu don't, it WILL lead to infidelity, in one form or another. Even if yeu do work it through, the solution may be... infidelity. It's not an inherantly bad thing as long as yeu understand the concept of why it occurs and treat it carefully and with respect to one another, and are open and honest about such. If it's agreed upon then honestly it's not a big deal at all. The only time it goes bad is when either one side refuses to help the other in any way shape or form (in which case they earned it by their own actions and lack of caring), or when it's done without telling the other it occured (in which case it's yeur fault for doing it and not giving them the option to be part of the solution, they can't show they care if yeu don't give them a chance, stupid).
Therefore, I will state that I am as faithful as humanly possible on the emotional, intellectual and sexual aspects... and will give yeu the opportunity to cover any gaps that yeu may not already be covering on yeur own. If yeu don't care enough to help though, then don't expect me to care enough to put up with yeur inadequacy. If yeu care, yeu can work towards a solution, whotever it is. If yeu won't even try to put effort in then yeu deserve nothing more than loosing that part of me. Do it enough, and yeu loose it all. This's yeur decision, as much as it is mine.
The only time when I'm totally 100% against infidelity, is when there's no communication, and a 'decision' has been made without even realizing there was a question to start. Trick questions, forced answers, and so on, these don't count. Yeu have to inform them of the issue, and make sure to work with them on the decision making process. If yeu don't do that, then yeu screwed up and they can't be held accountable for not living up to an expectation they didn't know they were supposed to.
I guess there is the whole - if I'm not payign attention, no one else will.
