Being "in love" usually keeps a person from wanting to hurt or betray another. Feeling neglected, taken for granted, verbal/physical abuse, and rejected sexually can lead one to explore other "options" or at least cause them to be more receptive if shown some appreciation by another.
Aside from "being in love", (Both people have to want the same things!) There is no right or wrong, only agree and disagree. If two people don't want the same things it's over.
It's also important to remember in all relationships we are either "growing together" or "growing apart", "nurturing or neglecting".
Just because someone is married does not mean they get to "Relax". Much like the work in a garden a person must continue to plant seeds, water it, pull up weeds, and keep out the pest. Never stop dating, having sex, and being romantic!
There are bascially 3 types of cheaters
1. The incessant cheater ( gets bored easily, flirts constantly)
Motto: Variety is the spice of life!
2. The unbeliveable opportuntiy cheater
(Someone attractive/out of their league hits on them)
Motto: The best way to handle temptation is to yeild to it!
3. The discontented cheater (neglected/unappreciated)
This person feels "justified"
The first two comes down to selecting the wrong mate.
The third one however can be the result of not listening to a mate or ignoring their needs in some way. However it has the best chance of working out if both people are willing to make some changes.
It takes more courage to walk out of marriage than most people have. Given a choice between going through a costly messy divorce or venturing outside in attempt to fill a gap... A lot of people will opt to step out on their spouses.
Other people treat relationships like jobs... they can't leave one unless they know they have another one waiting for them in the wings.