You won't get a satisfactory answer, for she does not know herself.
Do not assume that she thinks adultery is OK; she is likely very much opposed to it, but right now it fills a need.
You are assuming that she is acting rationally and has justified all of this to herself. More likely she is acting totally emotionally and is at great conflict with all of it.
Remember when you were 17? Did you ever have anyone tell you that your girlfriend wasn't right for you? Did you listen? Resist the urge to reach out to question and reason with her. That is not something she is really capable of right now.
It can be a great waste of time to try to figure out why they do what they do. Even if you figured it out, there would be nothing you could do with the knowledge. Far better to identify what in yourself you can work on to make improvements. Identify your goals, work up a plan and start executing.
In your earlier post you said you were going to give her up to God and let go. Let God take care of her right now while you take care of yourself.