The goal of any cheater is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Most cheaters have no interest in (replacing) one relationship with another. They'd rather stay than go through a messy breakup.
I believe there are 3 basic types of cheaters
1. The Incessant Cheater
This person gets bored very easily. They love the excitement of being with someone "new". Their motto is: "Variety is the spice of life!"
Odds are this person has never been monogamous in any long-term relationship and they should probably avoid committing to exclusive relationships. Their main failure is not acknowledging who they are. Monogamy is a lifestyle choice and not some "strict diet" to try now and then. They're better off having "booty calls".
2. The Unbelievable Opportunity Cheater
Unlike the Incessant Cheater this person is not "pro-actively" looking to cheat. They may have lusted in their heart after someone, have a crush, reminisce about an ex or met someone whom is considered "out of their league" One day out of the blue the object of their affection hits on them! From their point of view it's a "once in a lifetime" opportunity. They cave into temptation. Oftentimes this cheater may later confess weeks, months, or years later to rid themselves of the guilt they've been carrying around.
3. The Discontented Cheater
This person blames YOU for the reason why they decided to cheat! They're "reacting" to something you stopped doing, or something you are doing. They feel emotionally disconnected, neglected, taken for granted, sexually frustrated, or invisible. Someone came along and gave them attention and made them feel "special" again! Most cheaters who get caught will claim to be in this group. After all it is the only one where the betrayed person (might believe) they contributed to their mate's decision to cheat and therefore they may be incline to forgive them.
The cost factor sometimes comes into play. Imagine an unhappily married man whose wife stopped having sex with him. He could run down to the courthouse and file for divorce, move out of his home into a condo, pay alimony & child support, and become a weekend dad or he could cheat. Bear in mind most cheaters don't expect to be caught! Odds are he'll cheat! The old adage is: "It's cheaper to keep her."