If it's possible to become an ex smoker, ex drinker, or ex drug user then logic would suggest it's possible to be an ex cheater. Essentially both monogamy and cheating are a (choice) one makes and not a DNA code.
People cheat because they (want) to.
For anyone new to the "80/20 rule for relationships," it's the theory that says, in a fairly healthy relationship you only get 80 percent of what you want.
The goal for most cheaters is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side. They are not usually looking to (replace) one relationship with another.
They want to "compliment' what they already have.
Most cheaters refuse to go without the "missing 20%".
Whatever is "missing" clearly does not rise to the level of being a "deal breaker" in their eyes because if it did they would end the relationship. There are 3 basic types of cheaters.
1. The Incessant Cheater
This is someone who has never been monogamous in any long term relationship. They get bored very easily and always on the hunt for the "thrill" that comes with being with someone (new)! Monogamy for them is the equivalent of going on a 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" on someone like a co-worker, a stranger they see at Starbucks or whatever. One day the object of their crush actually flirts with them or hits on them!
In addition their mate may be out of town or they are the one's out of town for business and the odds of getting caught are dramatically reduced. It's a once in a lifetime opportunity to turn a "fantasy" into reality. This person caves in to temptation!
3. The Discontented Cheater
This person blames you!
If you hadn't done or stopped doing whatever; they never would have stepped outside of the relationship. They're saying (you) made them do it. If a betrayed person buys into this excuse they may be willing to forgive and work on the relationship.
The reality is people (choose) to cheat on their own in an attempt to have it all.
A betrayed person has to decide if (cheating) is a "deal breaker" or not. Each of us is entitled to have our own boundaries and "deal breakers."
Has the cheater shown genuine contrition, asked for forgiveness, is being transparent...?