Many times when someone cheats the person your significant other is cheating with is called a homewrecker. Even though it takes two to tango and he/she shouldn’t have gotten involved with someone in a relationship, they still did not wreck your home. If your partner is cheating on you then the relationship is already doomed. If someone gets to a point in a relationship where they are comfortable with being involved with someone other than you that means they don’t respect you, therefore the mistress is technically doing you a favor.
It’s no one’s fault but the cheater’s that the relationship got wrecked. If you get cheated on then you either are dating someone who never actually cared about you or your relationship has hit a patch where there are too many problems to fix, either way it needs to be ended. So it’s not the other man/woman that broke your relationship they are just the one that made it obvious that the relationship is broke.