There are two very opposite, very compelling arguements regarding this extremely common topic. Let us hear both sides of the story and then decide for ourselves.
If you are the “third person” in an affair, you technically aren’t doing anything wrong. You are single and having sex and having fun. If the person you are “having fun with” happens to be in a relationship, that is their issue and not yours. It’s not like you forced them to hook up; they made that decision on their own. Hell, they even seduced you. You know they are having as much fun as you are, and even though this may be breaking up a relationship, it is not your fault.
You know full well that you are destryoing a relationship. How would you feel if you found out your partner was cheating on you? You would probably want to wring the necks of both your partner and the “cheatee”. Knowing this, how can you have a proper affair? You are being a huge hypocrite and your actions are extremely disgraceful. There are millions of men/women in this world for you to start a sex dating relationship with, why do you have to choose and be with someone who already has a significant other?
If you are going to sleep with someone who is taken, you should know better, but at the end of the day it is their issue and not yours.