I would say, based on my work with couples, (1) that the reasons are many and varied; and (2) that the reasons for males and females who cheat are pretty much the same.
dissatisfaction with the marriage - the biggest factor
anger at the spouse/partner
desire for excitement
desire for novelty
need for reassurance and affirmation
fears about growing older
These are probably the big ones.
Don't forget that in spite of all the publicity given to cheating in relationships, there are still many people who don't cheat. It's easy to become cynical about humanity reading the newspaper or magazines but there's another side to it as well. A successful (and faithful) marriage isn't usually news - only the unsuccessful ones get talked about.