Cheating is unacceptable whether married or in a relationship. Here are some factors to know why people cheat and what makes them more capable of cheating.

Cheaters cheat. That’s why it’s inadvisable to marry someone who was cheating in an affair with you—when you’re the spouse, you’ll get cheated on. People who feel entitled to sex any way they can get it, will always rationalize cheating, and just keep doing it. They don't want to work on marital issues, or learn to keep intimacy alive with the same person. They get their jollies from illicit sex (cheating.) If sex and intimacy go out of a marriage, the likelihood that cheating will happen increases. Men tend to cheat because they are not sexually satisfied, and feel entitled. Women tend to cheat because they don’t feel emotionally connected with their husbands or partners, and find someone else (a training coach, mentor at work) who is supportive and comforting.

Here are some factors that make someone more likely to cheat:
1. Dissatisfaction with relationship: When men feel entitled to sexual satisfaction, and they are not getting it, they tend to cheat. Women will tend to cheat when they feel emotionally disconnected or rejected. There are people who choose to remain in an unsatisfactory relationship for a number of reasons, like children, finances, or family, and make up for what’s missing with an affair.
2. Past habits of cheating: If your partner has cheated before, on you or another partner, he or she is much more likely to cheat again.
3. Availability: Partners will cheat more easily if the person they have a connection with is readily available, such as someone at work, or a close neighbor or friend. Cheating can arise from a growing emotional connection or infatuation if both parties are willing to cheat. 
4. Family history: People who grew up in an atmosphere of betrayal and cheating are more likely both to cheat and to select partners who are unfaithful. Sometimes, anxiety about being cheated on can motivate a partner to cheat.
5. Distance: Physical distance, such as long distance relationships or jobs that require a lot of travel, can promote cheating. Both the loneliness factor and the opportunity of being away from a partner can be motives.

It’s very often possible to fix what’s not working in a relationship, if both parties are willing to work at making things better. Marriage counseling can be surprisingly effective, even once cheating has occurred.