Please note that I am not trying to make excuses for men. Cheating on your wife is wrong. Here we will try to understand and in some cases explore methods for managing, a cheater’s behavior.

The numbers
Current studies indicate that around 52% of US men and 44% of women have cheated on their spouses. It is worth noting that the number of women, who cheat, has steadily increased over the years. This indicates that the number of cheaters is determined by how permissive society is, rather than the sex of the cheater. It is worth mentioning that women cheat for different reasons than men and that when they cheat, it is usually with fewer partners.

We can see more of this social influence, in that the third most often given reason for infidelity is that the cheaters friends are cheaters. There for he felt that it would be ok to go ahead and try it.

Emotional needs
This is the number one reason that men give for cheating. Frequently you will find that the cheater’s wife is sexually more attractive than his mistress. In this case the mistress makes him feel like ‘a real man’, while his wife and family don’t. She does so merely by acting and speaking to him in a way that builds his male ego.

Sex is much more important to men, than woman realize. The quality of a man's sex life seriously affects how they perceive themselves and their relationships.

Sexual need
If you are having sex with your husband rarely or not at all, chances are that he will go looking for it elsewhere. If this is the case, it’s not a matter of the quality of the sex, just the quantity. Perhaps you might consider try to chemically balance your needs in this area?

Sexual boredom
Men like to try new things in the bedroom. If you aren't willing to experiment, a bit, they may seek this excitement elsewhere. It shouldn't be hard to find things that are new, but still within your boundaries.

Seeking sexual excitement outside the marriage is connected to a husband's social circle. If they feel their friends are going outside their marriages for exciting sexual experiences. They are more likely to try it themselves.

Revenge
It doesn't necessarily mean that you cheated on him, now he cheats on you to get back at you. It can revenge for something else you did.

They think they can get away with it
If they think you will not punish them for it(maybe because you didn't do so in the past) or they think it is unlikely that they will get caught. Then men are more likely to cheat.

They are very unhappy at home
If you argue a lot and he seems to be under too much stress, he might escape from his problems by looking for a mistress.

I know this article does not fully answer the question: Why do men cheat on their wives, but I hope it has given you some insight into how it happens.