Most Helpful Guy
Both men and women cheat because they are unsatisfied, and for some reason are unwilling to leave the relationship they are in.
A small set of men and women (more men in this category, I believe) are serial cheaters who find it difficult to be sexually expressive with someone they are emotionally intimate, or, have a very high need for multiple partners but for some reason still feel the urge to be in a monogamous relationship. These are a minority of people who cheat, but they cheat a lot, so a lot of people have contact with this behavior.
More often we have people who are deeply unhappy but can't or won't leave. I see more cheating in marriages then dating, because if people are that unhappy dating, splitting up is easier. Some people don't because they are afraid, afraid of being alone, or afraid of the challenge of ending things. They cheat 'on the way out' of a relationship. When people are more intertwined - married, sharing assets, living together even, certainly having kids - splitting up starts becoming very, very difficult. For most of human history, cheating was quietly tolerated while divorce was unthinkable. I will simply note that intelligent people have sometimes believed cheating was preferable - morally - to splitting up a family.
Women tend to describe feeling neglected and enjoying the attention of an affair, which leads to sex. Men tend to describe feeling sexually unwanted and ignored, and this leaves them very vulnerable if someone suddenly DOES express great interest. In fact, there's big overlap here in feeling neglected/wanted/appreciated/sexual, but men and women tend to label the driving factor differently. But its just looking at the same thing from another angle.