I believe that there could be many reasons, and that they are mostly quite subjective(depending on the person).
However, there is one reason which doesn't depend on the person so much, and that is attraction. While partially being able to control it with our minds, males will always be attracted to other females.
It just depends on how strongly we are willing to dedicate ourselves to this one person. Sometimes, in times of desperation or "need", men will cheat on their wives, sometimes... they are just simply what they call jerks. I'm sure there is a large number of reason, but... as i'm not planning to go into the "cheating business" any time soon, i don't really know many reasons why people would do it(except that they have maybe chosen the wrong woman to be their wife, and they can't admit it).