<human>: The other night at a bar, my girlfriend was dared by two guys to make out with another women, contingent on them kissing. She agreed to it, and the continued to make out with another woman. I was in the group as well. When given this challenge, she looked at me, we made eye contact and I shook my head, communicating I would prefer she didn't. After doing so, I told her it felt she cheated on me, I closed out my tab and went home. 

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Fast forward 2 days. We have spoken for several hours, she at first claims it doesn't count as cheating because "its every man's fantasy." I make it clear I feel differently about the situation than she does, I tell her I want to break up. She explains it means nothing to her, as she is not bisexual and convinces me to give it some time to think over. Every time i think about her I get sick. I can't even imagine ever taking her back or considering her to be a romantic component in my life anymore. I don't want to lose her as she has been my best friend for a year before we started dating, but I don't think I can let her back in to my life as if nothing happened. 

This experience really changed how I feel about this woman, I understand we are in college and it may seem appropriate to fool around and have fun, but in a relationship there are boundaries that must be respected.

Could you distill the essence of the text into 1-3 sentences?
<bot>: Girlfriend made out with another woman, I feel like she cheated on me, even though she doesn't think she did. Advice?