Just as "...but I love you" isn't a get out of jail free card following betrayal, "...but you cheated" isn't either. In both cases it is a deflection from the things that matter.
SpaghettiSam asks, "What has changed in this person so that the next time a crisis occurs or they aren’t feeling loved and special they don’t opt to go fuck another person?"
Labeling someone a slut - or a narcissist, predator, or whore - isn't rape, but the emotional intentions are the same. It's about taking someone's Power.