I imagine it's fairly easy to love someone and cheat on them.
There's a major difference between "love" and "in love".
When you love someone you care deeply about them but when you're "in love" you don't want to do anything that might risk you losing them! You don't "have time" to be (bothered) with whatever else is out there.
I believe there are 3 reasons why people (don't) cheat.
1. They are "in love".
2. They don't want do anything that would put their relationship at risk.
3. They know how hurt they'd be if their mate cheated. (Avoid bad karma).
It's been my observation that people cheat in order to STAY or tolerate their present relationship/marriage. Very few people cheat in order to "replace" one relationship with another.
Their goal is to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side.
Sometimes it's about economics and living standards.
Imagine a married guy whose wife has no sexual interest once menopause set in. They've had discussions and tried some other things but she still has no interest. From his point of view he now has three options.
1. Give up having sex for the rest of his life.
2. Run down to the courthouse and file for divorce, move out of his home into a studio apartment, pay child support & possible alimony, become a weekend dad, and divide up family and friends as they choose sides.
3. Find a woman who is sexually attracted to him and physically desires him.
Many men go with option #3. Cheaters don't expect to be caught!
In no way am I "justifying" cheating but you asked why people cheat.
Clearly they had the option to breakup or file for divorce. The reality is they chose to cheat in order to tolerate their existing circumstance.
Naturally one also has to look at the age of the person as well. Cheating while in one's teens and 20s is often tied to immaturity, ego gratification, and not knowing oneself well enough to know they're not ready for (exclusive commitment). I believe there are three basic types of cheaters.
1. The Incessant Cheater - Gets bored easily; Motto is: Variety is the spice of life! (They will never be faithful for long).
2. The Unbelievable Opportunity Cheater (Caved to temptation/impulse at chance to make a fantasy reality. They may feel guilty and confess years later)
3. Discontented Cheater - Blames you! (You started/stopped doing something that made them feel no longer "special or loved")
Only you can decide if cheating is a "deal breaker".