My gf (18f) got drunk at a party and had sex with another man and thinks that I (19m) am taking this way too seriously.

My girlfriend went to a party the other night and got a little high and drunk. Whilst being drunk, she had sex with another man. She told me about this and tried to tell me that people make mistakes and it just happened. I obviously wasn't very happy to hear this from her and got very upset. She tells me that I'm taking it too seriously and how I shouldn't be mad because she wasn't "herself." She then proceeds to tell me that if she wanted to leave, she could have a long time ago but chose to stay because she wants to "be there for me"

Now, in my book, I classify this as cheating and she was very upset and told me that I'm gonna take it that far. But she just won't listen.

Am I really in the wrong for not being "understanding" and that I should just accept her mistake?

Appreciate any help and thank you for reading this post!

Edit/Update: I've cut ties with her. I was in denial and didn't want to believe her, but in the end, I can't change what happened. Shame that it has to be this way, but I can't have someone like her in my life.

Thanks for all the replies and help!