I highly doubt honesty and faithfulness is determined solely by chromosomes...
Both genders cheat. Whether or not one cheats more than another is anyone's guess.
The reasons (not excuses--there's no justifying it) people tend to cheat are:
1) Changes in quantity of sex.
Usually one partner's sex drive diminishes while the other's drive does not (or actually increases). This can be caused by stress, hormones, children, work schedules, activity schedules, etc.
2) Changes in quality of sex.
Let's face it: men warm up and cool down like microwaves and women warm up and cool down like crock pots. Constant 5-minute quickies do not a satisfied woman make...
3) Changes in perception of one another.
Men fall in love with their eyes while women fall in love with their ears. For women (and I know this will sound shallow), not keeping yourselves as physically attractive as in the past may alter the perception of the man. For men, going from being charming to farting in bed every night for a laugh is going to change the perception of the woman.
4) Boredom.
This is unfortunate, but it can happen, especially when one or both partners are predisposed toward low boredom tolerance (e.g.: people with ADHD or Bipolar Disorder).
5) One or both partners realizing that monogamy is not working for them.
Some people prefer the variety of different sexual partners, but are too ignorant to realize it before pursuing an intimate, monogamous relationship leading into marriage.
6) LACK OF COMMUNICATION!!!
BIG ONE! Passively letting #s 1-5 happen instead of talking about it won't help. Aggressively belittling one another because of 1-5 won't help either. Passive-aggressiveness is even worse...that just frustrates the other even more now that they have to decipher cryptic messages...
Be assertive! If the sex is drying up, ask about how the person's feeling. Encourage communication as well as consequences for lack of communication!