AITA for agreeing to participate in a polyamory relationship with my girlfriend and then immediately seeing other people?

I've been dating my girlfriend (24F) for 2 years. We live together and things have been great. A few days ago she asked if I (26M) would be OK if she started seeing other people, but not in a breakup kind of way, just seeing a few people on the side. She said she'd heard about polyamory from a friend and was curious. Being in a good mood and generally up for anything, I agreed.<br><br>That evening she and I went out to dinner and had a great time.<br><br>The next day I had lunch with a friend/coworker of mine, "Eileen" (23F). Eileen and I always got along and I realized part way through lunch that I might actually have a little crush on her. Old habits die hard and I was about to write her off when I remembered that I had permission to explore any other connections I might have with other people. I decided to roll with it and asked Eileen out on a date. Flirtation escalated quickly into a romantic/physical connection. It's honestly been a while since I connected with anyone like that.<br><br>This went on for almost a week before my girlfriend found out. I had been honest with Eileen up front (I always tell the people I date about my other relationships if they exist) and my girlfriend found out when asking Eileen a work-related question and being blindsided.<br><br>My girlfriend was angry and disappointed, but not at me for seeing other people. She was mad at me specifically for seeing someone we both knew. She thought that was "too close to home" and that it was bad form. I told her she was being unreasonable and hypocritical if she was going to put rules on what connections I could explore. We have been arguing about this for a while.<br><br>AITA for having a connection with someone my girlfriend knows?