Men cheat because it is an ego boost for them. Women are bombarded every single day with offers. Men on the other hand have to work for it. I heard comedian Chris Rock once say "men are about as faithful as his options are." That means if your man is Kobe Bryant he is a lot more likely to cheat on you than if your guy is a fat loser. You can combat this ridiculous notion by working to constantly build your guy up and make him feel like a king.
Genetics plays a big part in why men cheat. This is a common idea presented to the lady when the guy gets caught. "Well baby its in my DNA I cannot help it." I believe this is a total cop-out and should not be accepted at all. It is also in your DNA to do a lot of other socially unacceptable things and you don't do them. Do not in any way accept this as an excuse from your man.
Men cheat for the adventure of it. Infidelity can be an exciting rush to an otherwise mundane life. In this case the affair is more about the conquest than the prize. The chance of being caught is a thrill and they enjoy the excitement. Often this is coupled by seeing their friends who are cheating. At this point their cheating becomes a competition. To combat this, keep your relationship exciting. Be spontaneous and live life a little more dangerously so that your man doesn't see you as boring. As for the competition aspect of this. If you feel your man's friends are cheating you should talk to him about it. Ask him how he feels about what they are doing. It really doesn't make much sense to go talk to their girlfriends or wives. Truly this is about your man not anyone else.
Men cheat for a variety of reasons. These are just a few and there is a lot more to each of these. If you fear you are being cheated on, or you have been cheated on, these are not excuses. These are simply reasons why men hurt the one they love the most. Ultimately you have to decide what you are going to do about their infidelity.