No one has to cheat!
Both monogamy and cheating are (choices) people make.
In addition if someone (wants) to cheat on you they will.
I believe there are three basic types of cheaters.
1. The Incessant Cheater
They become bored very easily and long for the excitement that comes with being with someone (new). Their view of monogamy is equivalent to going on a very strict diet. It's not a matter of (if) they will cheat but (when) they will cheat.
Their motto: "Variety is the spice of life."
2. The Unbelievable Opportunity Cheater
This person is not "proactively" looking to cheat. They may have a secret crush or someone believes a person is "hot" or whatever. However one day this person hits on them! (flirts, or makes it known) they could turn their fantasy into a reality. Circumstances might also be where their mate is out of town or they find themselves away on a business trip and odds are they won't get caught.
This person caves in to temptation.
Sometimes this type of cheater has been known to confess weeks, months, or years later to remove the guilt they have been carrying.
3. The Discontented Cheater
This person blames you!
If you hadn't stopped or started doing whatever it is they never would have stepped outside of the marriage/relationship. Essentially they feel you stopped making them (feel special), you took them for granted, always complain but never complement, pulled away emotionally or physically...etc
I assume this is the type of cheater your question is concerned with.
Sometimes a medical condition or natural transition of getting older such as menopause might effect one's libido or interest in sex.
Oftentimes in such situations the person with the higher libido is made to feel selfish for wanting to maintain the sex life they were accustom to having. If their mate is unwilling to explore a variety of ways to please them there is a risk they might be open to finding someone who will address those needs.
However just "talking" will not curtail their sexual desire.
In some instances where a person finds them self paralyzed or bed ridden and simply unable to have sex they might decide to "compromise" for the sake of continued love and companionship by looking the other way or allowing their mate to have discreet trysts.
Everyone is entitled to have their own "deal breakers" and if cheating is a "deal breaker" for one person and a "sexless marriage" is a "deal breaker" for the other person it's probably best that they move on. Unhappy couples are toxic!