No matter what we believe at any given time, our beliefs will modify to accomodate our actions.
As an example:
Person Z believes that they should be monogamous in a relationship
- Z cheats, feels bad, and still thinks they should be monogamous
- Z gets hit on again, 'somehow' ends up sleeping with X...feels terrible, and still thinks they should be monogamous
- Z has a fight with partner...cheats...and goes 'damn, well it wouldn't have happened if A,B, C'
- Z ends up cheating again, and goes 'well, I'm not a bad person, so it can't be too bad to cheat.
- Z does again, and goes 'you know what, I'm sick of feeling guilty'
Now that may not be the exact rationalising away of what they are doing, nor the exact path...the point is, that if you look at the start (should be monogamous), and look at the end ...5 affairs later...and the beliefs associated with their identify vs what they are doing - their beliefs have changed.
Then again, it's a little more complicated than that, because we have logical beliefs, and emotional beliefs (beliefs in different parts of our brain) that compete with each other...but again, some part of our beliefs change.
You can reverse that, when in a relationship, by not even flirting with the other sex...being friendly, but not flirting with anyone other than your partner:
- at each stage that you choose not to flirt with the other sex , you start reversing the beliefs that supported cheating
The problem is...cheating is supported at very low levels of the brain (the instinctive level). Instincts to can be trained - they just take longer. In other words, it would take discipline to stick to 'not flirting with anyone else'.
If that sounds too boring, then you are not ready for a monogamous relationship