Most Helpful Girl
Yes, Cheating is a choice, a choice you make because you neither respect or value your relationship, or the person it is with, enough to not to cheat. You are deliberately breaking the trust and agreement you have made with your partner, to be faithful, and to value heir well-being above or equal to your own. A relationship won't continue on a fulfilling trajectory after someone has hurt their partner in such a deliberate way (no cheating is not an accident), and of course, trust has been broken, so the bond between the two of you, and the purpose for your relationship - which is to be with each other only, as opposed to anyone else - is no longer the case.
Most Helpful Guy
If it was intercourse, oral sex, fingering/hand job yes I would dump them regardless of their reasoning. If it was kissing and they were upfront about it and regretted it I would find out why. If it was a weak moment because of something that happened in the relationship then I would try to talk and understand but tell her to never do it again as it hurts. If it was a drunken mistake then yea I'd probably seriously have to consider ending things or at least give her some rules to follow regarding parties. My ex kissed a guy once because I was causing problems in the relationship; she was drunk and told me but I handled it poorly and it caused even more problems that led to our break up which I now regret. Sometimes you need to try and see things from other peoples POV