Was I overreacting or was it a good thing to do?

Girlfriend asked if we could have a threesome with a girl friend of hers. As a 20 something at the time I thought i hut the jackpot. Was an awesome experience but we set up certain rules which I thought was smart. Ofcoarse after we had done it with her friend she brought up wanting to try two guys. As it was only fear we gave it ago. Same rules applied.

However a week later I found her breaking those rules, contacting the guy and organizing another meet up. She said I was being to much of a prude and to get over it. I found out later she'd slept with other guys before any of this happened and I decided to break it off.

Was I overreacting or was it a good thing to do? Rules were rules to me.