When a man cheats, chances are he just wants to get his rocks off with a sexy/ alluring/ hot girl who he sees as sufficiently varied from his girlfriend. Say his girlfriend has brunette hair and a tomboy look. So he might cheat with a leggy feminine blonde. Same goes for the type of sex on offer. If his girlfriend is quite "vanilla", he may look outside the relationship to get his kinkier urges satiated.
So for the man, it's about physical/ sexual urges, and they're strong enough that he abandons his loyalty to his girlfriend for the possibility of 'quenching his thirst', as it were.
When a woman cheats, the causes I believe are a bit more complex. So far I can tell, it usually comes down to her wanting to experience some specific emotion-- or range of emotions-- that isn't on offer within her relationship. For instance, her boyfriend may be dependable and give her a feeling of safety and comfort. So she might cheat with a "bad boy" who represents the emotions of danger, passion, aggression, and unpredictability.
So for the woman, it may partly be just pure sexual urges, but the bulk of the desire to actually sleep around stems from the *emotional variety* she hopes to derive from the affair.
This also tells us something about the nature of how cheating plays out *over time* for both men and women.
A man who already cheated may continue to cheat until the newly acquired sexual variety "wears off" and loses its allure. He may just want to "get it out his system". And because men tend to compartmentalise their inner thoughts/feelings, he will probably view this as not having any reflection on his feelings for his girlfriend. In fact, he may claim to love her just as much-- and I don't think he's lying when he says this (even though the contradiction is apparent to any outsider looking in on his inner dialogue).
A woman who already cheated may continue to cheat until the emotional variety is no longer present. Maybe part of it was the thrill of getting caught, and the thrill of being "naughty" by doing something taboo and "wrong". If that's all the emotion driving the cheating, then it can't last forever surely. But if she's actually formed a genuine emotional attachment to the new lover, then she will most likely continue to rationalise extending the affair, pretty much indefinitely, unless something forces a change in her situation (and even then, the *desire* to go back to her lover may only intensify). After all, we've all heard plenty examples of women who cheated on husbands for 10+ years. But when the man cheats, it's far more likely to be a brief "fling", or series of flings with different women.
This is, I believe, why men often take getting cheated on as more of a betrayal than women. Because a man cheating "means" less in many cases, since he's usually motivated by plain simple lust. But for a woman to cheat, all kinds of emotional needs must first go unfulfilled in the relationship, and then she must locate a new lover who represents all those unfulfilled emotions, in just the right proportion. So the decision to cheat for these women may be "slower", but once they do it, they're more committed to continuing the affair, and all the necessary lies which sustain it. The decision for a man to cheat is more like a gas canister being slowly pressurised until it "pops". Once that sexual tension is released, he may go back to his partner without giving the side chick a second thought.
Disclaimer: I fully believe that cheating is ALWAYS a conscious decision of the cheater, and so the rule of personal responsibility is never exempt. Once you decide to cross that line, you can't uncross it, and you disrespect both yourself and your partner at the moment the decision is made. My intention here is only to note that intrinsic motivations differ, and that the fallout of cheating will itself be modified by those motivations.
Now, there is a positive takeaway from all this:
Even though motivations to cheat may differ between men and women (generally speaking)… the fundamental reason for cheating is that the cheater has a NEED which is currently unfulfilled, and which they're willing to go seeking outside the relationship.
So if we follow this reasoning to its logical conclusion, the lesson is clear: if their needs are satisfied INSIDE the relationship, then the motivations to stray will never arise in the first place, and therefore the cheating will likely not happen.
(Of course, you could argue that "a cheater will find a way to justify cheating one way or another". But I doubt most cheaters are "born" to cheat. Most cheaters were perfectly loyal, loving partners-- UNTIL THEY WEREN'T.)
So if women want emotional variety, then bring that to your girlfriend inside the relationship. Mix in a little "naughty" along with your "nice". Develop your personality so you can bring out a range of emotions in her. Realise that sometimes she even may want the "negative" emotions like jealousy or anger (even if she won't admit it)-- so find a way to let her experience those in a "safe" environment.
And so, if men want sexual variety, then bring that to your boyfriend inside the relationship. Buy new sexy lingerie (Primark doesn't count). Explore your kinky side together. Let him dominate you in bed in new ways-- or tie him up, climb on top, and flip the script altogether. Try new sex positions. And when you see him check out girls in the street, take it as 'field reconnaissance' to find out what caught his eye, and add it to your repertoire. If his eye lingers on the girl in the bright red dress, then wear one of your own to the next social event you attend together, and watch him collect his jaw off the floor. And flirt with him as much as you can-- don't get complacent and let the sexual tension fizzle out.
I'd love to hear anyone's thoughts on this subject. I think the hardest part getting cheated on for me was being confronted with the idea that she was going to another guy for emotional fulfilment rather than myself. That hurt more than the physical facts of the "whats" and the "wheres" and "hows". I'm curious what it's like for girls to get cheated on. Did he cheat on you for sexual variety as I posited, or was it a full blown emotional affair as well? Does the difference even matter to you-- is "cheating just cheating", no matter what the motivations are?