Why you cheat, or why you would cheat, depends very little on physical attraction.
Recently another client who had cheated - and felt emotionally distraught and confused about - it came to me for counseling. Although these situations make me feel bummed and I don't condone cheating (meaning non-monogamy that isn't agreed on in advance), I have a lot of empathy for the humanity in breaking the boundaries of one's relationship and feel a lot of forgiveness for anyone who makes a mistake and wants to own it and rectify it. You and I all have ways in which we betray our sense of integrity and honesty, and this is a tough example that gets a lot of bad press.
In addition to the care and interest I feel for all of my clients, I really enjoy working with these men and women, partially because I find the discussion of monogamy and non-monogamy fascinating, and also in part because it's been kind of a research study for me, the details of which follow.
On the surface, cheating seems fairly simple. You, I, whoever finds someone sexually attractive and desirable enough to forgo the boundaries you've created in your relationship for that sexual gratification with this hot person. Maybe you're bored with your partner. Maybe he or she annoys you a lot now. Maybe he or she doesn't seem as hot to you anymore. And here is this new, shiny, sexy person who is drawing you in and seducing you more and better than your partner does.
I think it's fair to assume that some cheating is purely sexual attraction, kind of like I'm trying to present above. I mean, if I imagine Kate Upton or Devon Brugman were standing naked in front of me, it's simply tough - as a human that still, for the most part, has decision making skills that are about 80 percent the same as the instinctual animals that we see in the woods - to say no to someone that objectively sexy. (Insert female version of this point here. I don't want to degrade anyone here who's into men by assuming I know who the hottest men in the world are.) Nevertheless, many of the people who I've worked with who cheated will admit to me that the person who he or she has cheated with, or wants to cheat with, is less attractive overall than his or her original partner. So where does this fit?
I've noticed a pattern in cheating. People who cheat (ones who I know or have worked with) invariably do not feel accepted entirely (loved for who they are, however you want to say it) by their partners. They feel that their partners do not understand them or see them completely, and/or if they did they wouldn't be okay with it. Generally, they believe that their partners wish they were different in some way.
If you think about it, this makes perfect sense. If you feel someone doesn't completely get you or accept you, that part that therefore isn't okay will remain dormant and pent up inside. Yet, because for whatever unknown reason people have a need to be seen and accepted by themselves and others for all parts, these repressed aspects will come up for attention. And a good, sure-fire way for attention and validation is sex. What occurs to you or I as sexual attraction in the moment will in many cases actually be a need to be accepted or seen. (If you're scoffing at this because it sounds like new-agey nonsense I understand, but unfortunately I am correct.)
See, the human mind and consciousness are incredible, and sex in our day and age is much more complicated than the biological need to procreate. Our minds link together things that don't necessarily go together too well (for more on this read about heuristics and priming in the brain), and we end up feeling sexually attracted to someone for a reason that isn't all that sexual. Every human experience is very complex, but sex is one of the most complex because of how ingrained it is into our culture, and therefore daily life.
The point is that if you feel the need to break the stated boundaries of your relationship by having sex with someone else, or some other action that could be considered a betrayal, the first place you should look is at the ways you don't feel accepted by your partner. If you can't find anything here, look for the things that you believe they'd be okay with, but you hold back anyway because of your own fear. (There's a couple sub-points to note here: 1) you can make up things that you think your partner won't accept, so it's your job to just be honest and then they can deal with their shit, and 2) it's also important to create a place in your relationship to understand, if your partner has expressed concern about a part of you, why this is the case for them - what is it about this action in you that they fear?)
A successful, long-term relationship is one in which both people are willing to be entirely revealed. If this is not the case in your relationship, you must work on understanding and being understood. Try to find the gaps in the parts of you and your partner that want to be heard, revealed, understood, cared-for, but are not. I promise you that if you take the risk and tell the truth, the relationship will be brought to a whole new level of love. (Or you will find out that your partner actually isn't a good partner for you and you can consciously move on. Either way, I think that's good.)
The desire to hide begets an unfortunate but relatively common relationship goal: to keep it going. Right along side that goal is the goal to be thought of as good, or be liked or loved by your partner. The problem is that if you try to be loved for only some parts of yourself out of fear that the whole of yourself will cause conflict or dislike, those rejected parts will end up making their way out with someone else or in some other situation.