//Boxy - you may indeed question why people have an open relationship, but surely you are mature enough to know that not everyone subscribes to the prescriptive life that society maps out for us - monogamy, marriage, kids...?//
I don't question other people's lifestyles at all - each to their own, you would find me accepting of any sort of relationship patterns, and always have been. Mature is a strange word to apply - IMO, this sort of knowledge doesn't come with maturity, more with experience. Of course not everyone subscribes to a norm, we are all different, but the expectations of any long-term partners in a relationship should be the same. Yours and your partner's clearly aren't - he sounds a one-woman sort of guy.
My question about open relationships was more as to why you'd entered into a relationship if it would tie you, but you've since explained that you don't live together, and IMO not being live-in partners can make for a looser relationship than if you share a roof. I've tried many models of relationship during my life - I know what suits me now - it sounds as if you are still looking.