The goal of most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side. Essentially they are looking to compliment what they already have.
Whatever the cheater feels is "missing" in their relationship doesn't rise to level of going through a breakup or divorce over.
Very few cheaters are looking to "replace one relationship" with another.
Because cheaters don't expect to get caught they believe it's possible to "add" whatever they feel is missing to complete their happiness.
Having said that I believe there are three basic types of cheaters.
1. The Incessant Cheater
This person probably has never been faithful in any long-term relationship. Their view of monogamy is equivalent to going on a "strict diet". They know it is beneficial and yet at some point it's not a matter of (if) but rather (when) they will cheat. This person gets bored easily. They crave the excitement that comes with being with someone "new". Their motto: "Variety is the spice of life!"
2. The Unbelievable Opportunity Cheater
This person is not proactively looking to cheat. She/he may have a secret crush on someone or considers them to be "hot". One day that individual actually "hits on them" or makes it known they could hookup. "It becomes an opportunity to turn a fantasy into reality".
Sometimes immaturity and peer pressure plays a part in which a person is being egged on by friends or co-workers not to ignore the opportunity to be with someone this "good looking".Couple this with being on a business trip or circumstances where they are unlikely to be caught this person essentially "caves in to temptation".
It's not unusual however for this person to later confess weeks, months, or years later to their loved one if the guilt afterwards becomes too much to bear.
3. The Discontented Cheater
She/he blames YOU! Something you did or stopped doing caused them to be open to looking outside of the relationship/marriage. They felt ignored, taken for granted, constant complaints or arguments, loss of emotional or physical intimacy. Someone came along and flirted with them, put a smile on their face, and in their heart.
They felt "special" and "alive" again.
Ultimately it's the individual that chooses to cheat or be monogamous. Nevertheless if they believed the state of their current relationship was at the "deal breaker" point they would have left. Essentially they cheat in order to STAY or tolerate their existing relationship