I've read an awful lot lately about women who are cheating purely for the sex. They don't want to break up the relationship they are in, but they want better sex than they are having at home. They don't want emotionally involvement and if the guy they are sleeping with starts to get too attached they break it off and find somebody else.

https://medium.com/sensual-enchantment/women-want-great-sex-men-want-emotional-bonding-6dbfa8474c78

"One woman told me recently, “We’re polyamorous, but I’m primarily interested in sex. My husband wants to have relationships.” I hear versions of this same thing all the time."

So perhaps it's just different for different people, depending on the circumstances but you are certainly correct that cheating breaches trust but that a good relationship is worth saving in most cases.