She is a flirt and I want to break up...

My girlfriend and I were recently at a party where a guy began flirting with her. I was surprised to see her flirting, too. I didn’t say anything at the time because there were many people around, but I asked her about it later and she said it was harmless and she didn’t mean anything. I asked her if she would be okay if I was like that guy and flirting with some other woman, and she said she would have a problem with that. When I pointed out the double standards, she said I would have to accept her the way she was because I was stifling her personality. I find this hypocrisy difficult to handle and have begun to think about ending this relationship. She says I am making a big issue out of nothing and wants me to calm down, but I think this is right for me. Is this a rational decision or am I over-reacting?
Double standards of any sort imply an imbalance in a relationship. This may be her personality and she has every right to behave in a manner of her choosing, but you have the right to be offended too because your feelings are your own. This isn’t an overreaction as much as a decision based upon how you see yourself in this relationship. If she doesn’t get that, you may both inevitably have other problems down the road that won’t be resolved. Successful relationships work around individual personalities and are interesting because two people who are different try and get together. If she isn’t willing to change something that bothers you, you are probably making a decision that is right for you, even if the issue appears to be a trivial one.

Should I tell my girlfriend that I thought about having an affair when we had a huge argument?
What do you hope to accomplish by that admission, other than hurt her? What you should think about is the fragility of your relationship that made you consider this.