Although I don't condone infidelity it happens, and some might say evolutionarily defensible.In my opinion there are 2 distinct time frames of infidelity.
The new relationship, and the established relationship.Established relationship with passion on the downside.
Lack of Sex: I have to tell you ladies if your significant other is not having sex with you, you're in trouble. Short term set backs aside, an extended period of time with out sex is a good sign that something is amiss. Regardless of the reason for partaking in pleasures of the flesh with a spouse, humans are sexual animals and need sex. There is a good likelihood a man will go elsewhere to get laid, if the opportunity arises.
Nagging: You know if you get punched in the shoulder once it doesn't really hurt, but if you get smacked a million times it starts to smart a little. A woman that repetitively nags her man, for whatever reason, runs the risk of infidelity. A constant stream of digs and derisively conveyed needs and wants does little to bolster a man's desire of a woman. Communicate and talk about how you feel, don't just say "Clean the Garage, you bum."
Lack of Affection: Men are big babies, and in many ways are still the boys they were when they were little. Just like little boys need to be hugged, men need affection. Not just sex, mind you, but the little things, the reaching over to hold his hand. Leaning your head on the shoulder to let him know your care. Reach out to him, let him know you love him, want him, kiss him. Tell him these things. Men don't take hints. Dinner on the table, although appreciated, isn't as poignant a sign of affection as walking up to your man grabbing him, kissing him and telling him you love him. Men don't take hints.
Lack of Ego Stroking: Most men have big egos, not that I'd know anything about that. We need to be told how great we are. Not just great in bed, not just a great man, not just how attractive you think we are, but the simpler things are just as important. Bolster our egos with thoughts of us being good fathers, and thoughtful husbands. Tell us you desire us, and why. Make something up if you have to. Stroke our egos, and we'll not have to look for someone who will.
Lack of Communication: Believe it or not there are woman who don't like to talk about their feelings, or find it difficult to do so, just like there are men. It is important for a couple to share not just how they feel both good and bad, while keeping an open mind that what is being shared are just feelings, not then end all be all of what makes a couple tick. If you establish lines of communication early on, it facilitates talking throughout a lifetime of ups and downs that everyone experiences.
Letting Yourself Go: "My ex used to have an ass you could crack an egg on, now you can have a whole breakfast." A disturbing joke, but appropriate in many ways. Long term relationship cause both parties to take eachother for granted. There is no reason to look good to attract someone else, consequently the wife busy with the kids, groceries, and the day to day grind will let herself go, and the hubby no longer finds her attractive.
To Feel Young: As a man gets older, and he begins to realize his mortality, he will seek out youth in the form of female companionship that he thinks will make him feel young again. "You're only as old as the woman you're sleeping with."
Falling out of Love: After a long enough time, people if not careful, and communicating, will fall out of love. The desire, passion, and allure that was once commonplace in a marriage is no longer there. The love and passion can be rekindled, but sometimes it appears easier to just look somewhere else for that newness. As opposed to ending the marriage that is such a part of their lives, the man just cheats.
Variety: Variety is the spice of life. After a long enough time any relationship isn't what it once was. Something needs to be done to spice it up, games, romanticism, dress up, to change things from the same old, same old to something, ANYTHING different. This requires care, communication and understanding. Resting back on your laurels expecting something different and exciting isn't enough. A man will stray for variety.
Early on in a relationship, say the first 6 months.
Fear of Intimacy: Some men will cheat on a girlfriend as opposed to show their vulnerability and commit. There is a ton of control in breaking up by cheating. It is a true fear of intimacy that can cause this. They also will keep themselves emotionally distant from the mistress to avoid intimacy all together.
Sexual Incompatibility: The first few times having sex with a woman can be a number of things, crazy passion, new and exciting, but sometimes it can be two different people learning each other's bodies. There can come a time when the sex is good, but the overall compatibility is uncertain. The physicality of size, desire and satiability between two people isn't always compatible. A man will stray if he doesn't think the sex is working, or is as good as it should be.
Too Clingy: There is a fine line between attentiveness and clinging. It's a tough road to follow, since we all want our attention, and need our space as well until we really decide we want to commit.
Unrequited Love: If you've ever been in spot where you were telling the person you were with, and not getting a response you know this one. You've seen it in movies, read it in books. One person is putting themselves out there while the other is afraid to commit, or express mutuality of feelings. At some point after listening to silence after expressing your love, a guy will wonder if he's doing the right thing, and might look elsewhere to try and buffer the possibility of heartache.
The Back Up: Most eligible bachelors might be seeing 2 or 3 women all at the same time. It's called dating, of course most women call this being a player. When dating, AND looking for a serious relationship if after 6 months you don't see it going anywhere it's time to move on. After a couple months of dating a couple women, one will win out. The woman that gets dumped is sometimes kept as a back up, if the next relationship is a bust.
Do I still have feelings for?: At some point in a relationship, even one going good, a man might think that there was still that connection with an ex, leading him to stray in a decision making situation. This could go either way, it might be the final piece of the puzzle to undeniably convince him the new girl is the one, or it can be the end of the new relationship.
There is also the man who is never satisfied, that always looks for the grass being greener on the other side. That purposely sets out to find and fuck. A womanizer, player if you will, a man that bores easily, and is filled with wanderlust. These are opportunists and if given the opportunity they will cheat. They don't really need a reason they just cheat.