Truthfully most betrayed people could care less (why) someone cheats. Asking (why) is usually a "rhetorical question".
Most cheaters are looking to hold onto all that is "good" in their primary relationship while addressing their other "needs" on the side. Very few cheaters are looking to (replace) one relationship with another. They want to add to or "complement" what they already have. Others do it for the "thrill" of it.
There are basically 3 types of cheaters
1. The Incessant Cheater
(They bore easily and always want something "new".)
2. The Unbelievable Opportunity Cheater
(They caved in to "temptation" during a unique scenario)
3. The Discontented Cheater
(They blame you! Something you did or stopped doing.)
Ultimately each of us must decide for ourselves if cheating is a "deal breaker". It doesn't matter (why) unless you plan to stay.