Was what i did "unacceptable?"

During the last two weeks of my year and a half relationship i started developing feelings for another guy. But i never told the one i was dating at the time because i thought it would just be another crush that happens after not seeing the guy for 3 weeks. And i thought id get over it quickly. Not to mention, the boyfriend at the time told me " its fine as long as you look dont order"  So i just hung out with the guy to get to know him, and just to bevome friends. But the more i hung outwith him the happier i was. And so it made me think of the flaws of the relation at the time.  Thus i broke up with the guy. 
Following monday the exboyfriend calls me and he had found out about this guy. And he said " you didnt tell me? What if we were married? What you did was unacceptable and i cant trust you anymore" 
Its been a month since then and it still bothers me. We werent even close to marriage, he even stated several times he didnt believe in marriage. I never cheated on him. And i wasnt happy Anymore. 

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