Why Guys Cheat
Reason #1: SEX
Lets get the most obvious reason out of the way, now. It's not the most popular answer to hear from women, but, hey, it's honest. We cheat because we are not getting something sexual from you that we want. This could be anything from blow jobs, dirty talking, threesomes, or something out of the usual. This is law of supply and demand here. If we want the occasional (insert desire) every once in a while, but you refuse to give it, it makes the temptation that much stronger when "some skank" comes along who willingly will fulfill that need.
SOLUTION: Find out what your partner likes sexually, and if you're comfortable doing them, fulfill those needs. Make sure you're both honest and explicit. He may be into rape fantasies and S&M, and if you aren't, let him know. A disconnect sexually may be a signal that you two were not as compatible as you may have thought.
Reason #2: ATTENTION
This one is perhaps the most common reasons for cheating both for men and women. The classic example of this is the housewife who has an affair because her husband is constantly working or away on business. Although that is more common with women, we are all human and want to feel important and desired. Here is the worst kept secret of the male sex: we have very fragile egos. We want to feel important, powerful, and wanted. If we don't, we will find a way to fill that void. Sometimes it's a hobby, a new job, drinking, drugs, or the companionship of someone who gives the attention we were used to when you originally were together.
SOLUTION: Make it a point to stay in contact. If you feel the two of you drifting apart, take time to tell them how much you appreciate them and how proud of him you are. Make sure you are sincere with your compliments, and reassure him that, although things may be crazy and hectic, you still keep him in your thoughts throughout the day, and feel lucky to be with a man like him. Or you can really boost his ego by telling him how much he inspires you to be a better woman. A little attention, flattery, and affection goes a long way, and he will remember those words when some other woman starts swooning over how great he is.
Reason #3: YOUR INSECURITY
Although it's not a reason we should use to justify infidelity, being overly suspicious of your partner when he is being faithful to you can drive him into the arms of another lover. For instance, if you are the type to need to know where your man is at all times, text every 15 minutes when he's out with friends checking in on him, constantly berate every female that talks/looks/glances in the direction of your man, or repeatedly go through his email/phone/pockets questioning him on every unaccounted for piece of "evidence" you think you've uncovered, a guy is bound to get annoyed. I know it may sound terrible, but I've heard this statement more than once from guys: "I might as well go out and have some fun. She treats me like I do as it is already." Maybe not the best logic, but at the same time, I can't help but empathize.
SOLUTION: Back off a bit. Instead of constantly reminding him that you have your eye on him, focus on constantly reminding him how much you trust him and how happy you are he is honest and faithful to you. I'm not suggesting you turn a blind eye to warning signs and play stupid. If he is coming in at odd hours or getting unknown incoming phone calls, it's perfectly reasonable to ask. Notice I said ASK and not ACCUSE. The idea here is that when temptation presents itself to your man, you want him to be thinking of what a good woman he would be hurting, not what a mistrusting woman he would be fooling.
Reason #4: IT'S ALREADY OVER
The last one isn't a fun one to admit, but it's worth mentioning. Sometimes, guys will continue to stay in a relationship they know is doomed for failure, and continue to see other people behind their partners back. Most common example of this is perhaps a couple that got married young, had children, and grew into different people who can't stand each other anymore but keep it together for the kids. Unfortunately, it's more common that you think. Other reasons may be that the guy is scared of hurting the girls feelings by breaking up with her, keeping up appearances in social circles, and a relationship motivated by money (Gold-digging).
SOLUTION This gets tricky. . .specially when involving children in the mix. I advocate the solution that causes the least negativity, anger, and sadness in the long run. I would suggest to sit down with each other, and have a honest and blunt conversation on whether or not the relationship is healthy or worth saving. This may mean divorce or breaking up which will be very difficult in the near future, but it's better to go through months of heartache rather than years of lies, fighting, betrayal and possible health risks (STD's).
So, I've offered you four reasons that motivate a guy to cheat. Notice I'm saying they are motivators, not excuses. I have yet to hear a good excuse for why a guy chooses to cheat on a woman. In most cases it's an indicator of a relationship that was bound to fail soon enough. And yes, there are those guys out there who don't necessarily need a reason to cheat, they're just self-involved horny a**holes who live in the moment without regard for how their actions effect other people. However, assuming you are with a "good guy," avoiding the above four points listed above will drastically decrease the chances of your good man going bad.